Source : Webster's Revised Unabridged Dictionary (1913)

Clearstory \Clear"sto`ry\, Clerestory \Clere"sto`ry\, n. (Arch.)
   The upper story of the nave of a church, containing windows,
   and rising above the aisle roofs.

Source : WordNet®

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