The Collaborative International Dictionary of English v.0.48:

Clearstory \Clear"sto`ry\ (kl[=e]r"st[=o]`r[y^]), Clerestory
\Clere"sto`ry\, n. (Arch.)
   The upper story of the nave of a church, containing windows,
   and rising above the aisle roofs.
   [1913 Webster]

WordNet (r) 3.0 (2006):

clearstory
    n 1: part of an interior wall rising above the adjacent roof
         with windows admitting light [syn: clerestory,
         clearstory]