Error message? (4)
I believe the answer is:
'error message?' 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 "Exclamation on making obvious mistake" , "Exclamation on making a minor mistake" , "Exclamation on making minor error" , "I've slipped" , "Oh dear, that was careless!" .)