"What an idiot I am!" (3)
I believe the answer is:
doh
'what an idiot i am' is the definition.
(exclamation expressing recognition of stupidity)
This is the entire clue.
(Other definitions for doh that I've seen before include "for 7 [HOMER] of course!" , "an exclamation from someone who's annoyed with himself" , "Homer Simpson's exasperated cry" , "Tonic (mus.)" , "acknowledgment of stupidity" .)