The Collaborative International Dictionary of English v.0.48:

Undersong \Un"der*song`\, n.
   1. The burden of a song; the chorus; the refrain. --Dryden.
      [1913 Webster]

   2. Accompanying strain; subordinate and underlying meaning;
      accompaniment; undertone.
      [1913 Webster]

            In the very [poetry] there often an undersong of
            sense which none beside the poetic mind . . . can
            comprehend.                           --Landor.
      [1913 Webster]