Major screw-up (5)
I believe the answer is:
snafu
'major screw-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 "mess as usual" , "Mess table" , "chaos, as usual" , "in a bad state" , "Chaos (US slang)" .)