Source : Webster's Revised Unabridged Dictionary (1913)

Cleg \Cleg\, n. [Northern Eng. & Scot. gleg: cf. Gael.
   crethleag.] (Zo["o]l.)
   A small breeze or horsefly. [North of Eng. & Scot.]
   --Jamieson.

Source : WordNet®

cleg
     n : large swift fly the female of which sucks blood of various
         animals [syn: {horsefly}, {clegg}, {horse fly}]