"Didn't mean to do that" (4)
I believe the answer is:
oops
'didn't mean to do that' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for oops that I've seen before include "Exclamation after mistake or minor accident" , "Apologetic exclamation" , "Oh dear, that was careless!" , "Harem" , "The sound of stupidity" .)