Cry over spilt milk? (4)
I believe the answer is:
'cry over spilt milk?' is the definition.
(I've seen this in another clue)
This is the entire 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" .)