"Why did I do that?!" (3)
I believe the answer is:
doh
'why did i do that?' 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 "Homeric grunt of annoyance" , "Stupid me!" , "How foolish of me" , "acknowledgment of stupidity" , "First of the series at 17 Across" .)