The Collaborative International Dictionary of English v.0.48:

Clearance \Clear"ance\ (kl[=e]r"ans), n.
   1. The act of clearing; as, to make a thorough clearance.
      [1913 Webster]

   2. A certificate that a ship or vessel has been cleared at
      the customhouse; permission to sail.
      [1913 Webster]

            Every ship was subject to seizure for want of
            stamped clearances.                   --Durke
      [1913 Webster]

   3. Clear or net profit. --Trollope.
      [1913 Webster]

   4. (Mach.) The distance by which one object clears another,
      as the distance between the piston and cylinder head at
      the end of a stroke in a steam engine, or the least
      distance between the point of a cogwheel tooth and the
      bottom of a space between teeth of a wheel with which it
      engages.
      [1913 Webster]

   Clearance space (Steam engine), the space inclosed in one
      end of the cylinder, between the valve or valves and the
      piston, at the beginning of a stroke; waste room. It
      includes the space caused by the piston's clearance and
      the space in ports, passageways, etc. Its volume is often
      expressed as a certain proportion of the volume swept by
      the piston in a single stroke.
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

229 Moby Thesaurus words for "clearance":
   Lebensraum, absolution, acquitment, acquittal, acquittance,
   aesthetic distance, air space, allowance, amortization,
   amortizement, ample scope, approval, authority, authorization,
   back country, bill of health, binder, blank check, caesura,
   carte blanche, cash, cash payment, certification, cleaning out,
   clear space, clearing, compass, compurgation, consent, countenance,
   debt service, deep space, defecation, defrayal, defrayment,
   depletion, deportation, deposit, depths of space, desert,
   destigmatization, destigmatizing, detachment, disbursal, discard,
   discharge, discharging cargo, discontinuity, disculpation,
   disjunction, dismissal, disposal, disposition, distance,
   distance between, distant prospect, divergence, doling out,
   double space, down payment, drainage, draining, earnest,
   earnest money, egress, ejection, elbowroom, elimination, em space,
   empowerment, empty view, emptying, en space, enabling, endorsement,
   enfranchisement, entitlement, eradication, evacuation, excretion,
   exculpation, excuse, exhausting, exhaustion, exile, exoneration,
   expatriation, explanation, expulsion, extent, farness, fiat, field,
   forgiveness, free course, free hand, free play, free scope,
   freeboard, full pratique, full scope, full swing, gap, glade,
   headroom, hiatus, hire purchase, hire purchase plan, hole,
   infinity, installment, installment plan, interest payment, interim,
   intermediate space, interruption, interspace, interstice, interval,
   jump, justification, lacuna, latitude, leap, leave, leeway, length,
   light-years, liquidation, living space, long rope,
   maneuvering space, margin, mileage, monthly payments, never-never,
   no holds barred, off-loading, open country, open space, ostracism,
   outback, outlawing, outlawry, pardon, parsecs, pass, passport,
   paying, paying off, paying out, paying up, payment,
   payment in kind, payoff, permission, perspective, piece, plain,
   play, prairie, pratique, prepayment, protection, purgation, purge,
   purging, quarterly payments, quietus, quittance, range,
   ratification, rationalization, reach, regular payments,
   rehabilitation, reinstatement, release, remission, remittance,
   remoteness, removal, restoration, retirement, riddance, room,
   room to spare, rope, safe-conduct, safeguard, sanction,
   satisfaction, scope, sea room, separation, settlement, severance,
   single space, sinking-fund payment, space, space between, span,
   spare room, spot cash, steppe, stretch, stride, suspension, swing,
   terrain, territory, time interval, tolerance, unloading, venting,
   verdict of acquittal, vindication, visa, voidance, voiding,
   warrant, warranty, way, ways, weekly payments, wide berth,
   wide-open spaces, wilderness, withdrawal