I've messed up zero operations (4)
I believe the answer is:
oops
'i've messed up' is the definition.
(I've seen this before)
'zero operations' is the wordplay.
'zero' becomes 'o'.
'operations' becomes 'ops' (I have seen 'Military operations ' mean 'ops' so perhaps 'operations' could also mean 'ops').
'o'+'ops'='OOPS'
(Other definitions for oops that I've seen before include "Exclamation on making minor error" , "- - -a-daisy" , "My bad" , "Exclamation of surprise or apology when making mistake" , "Clumsy me!" .)