Cry over spilt milk? (4)
I believe the answer is:
oops
'cry over spilt milk?' 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 "Oh dear, that's a mistake!" , "Apologetic exclamation" , "Exclamation on making minor error" , "Something's been spilt!" , "Exclamation of making a blunder" .)