Head-slapping cry (3)
I believe the answer is:
doh
'head-slapping cry' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for doh that I've seen before include "First of the series at 17 Across" , "First note of musical scale" , "How foolish of me" , "whoops!" , "for 7 [HOMER] of course!" .)