United supporters flipped, creating a big mess (5)
I believe the answer is:
snafu
'a big mess' is the definition.
(I've seen this in another clue)
'united supporters flipped' is the wordplay.
'united' becomes 'U' (abbreviation).
'supporters' becomes 'fans' (fan is a kind of supporter).
'flipped' says the letters should be written backwards.
'u'+'fans'='ufans'
'ufans' back-to-front is 'SNAFU'.
'creating' acts as a link.
(Other definitions for snafu that I've seen before include "Bloomer" , "Chaos (US slang)" , "mess as usual" , "Mess table" , "Chaotic state, mess" .)