Big mix-up (5)
I believe the answer is:
snafu
'big mix-up' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for snafu that I've seen before include "Chaotic state, mess" , "Bloomer" , "Mess table" , "Chaotic situation" , "Slangy acronym meaning in total confusion" .)