"How foolish of me!" (3)
I believe the answer is:
doh
'how foolish of me' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for doh that I've seen before include "whoops!" , "for 7 [HOMER] of course!" , "First and eighth note in tonic sol-fa" , "an exclamation from someone who's annoyed with himself" , "acknowledgment of stupidity" .)