"What was I thinking?” (3)
I believe the answer is:
doh
'what was i thinking?' 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 note of tonic sol-fa scale" , "How foolish of me" , "Note; exclamation" , "acknowledgment of stupidity" , "First note of major scale in music" .)