Major mix-up (5)
I believe the answer is:
snafu
'major mix-up' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for snafu that I've seen before include "Slangy acronym meaning in total confusion" , "Customary confusion" , "a mess of things" , "mess as usual" , "chaos, as usual" .)