Military Intelligence wrong over mistake (3-2)
I believe the answer is:
mix-up
'mistake' is the definition.
(I know that mix-up is a type of mistake)
'military intelligence wrong over' is the wordplay.
'military intelligence' becomes 'MI' (as in MI5 or MI6 - British military intelligence).
'wrong' becomes 'x' (X is used to mark something as incorrect).
'over' becomes 'up' (e.g. if your time is up it is over).
'mi'+'x'+'up'='MIX-UP'
(Other definitions for mix-up that I've seen before include "Mistake caused by misunderstanding" , "Confused jumble" , "State of confusion or misunderstanding" , "Confused situation" .)