My mistake! (4)
I believe the answer is:
oops
'my mistake' is the definition.
(oops may be said to acknowledge a mistake)
This is the entire clue.
(Other definitions for oops that I've seen before include "My bad" , "Silly me, I've dropped something!" , "The sound of stupidity" , "Blunder explanation" , "Exclamation on making minor error" .)