Chaos upset United supporters (5)
I believe the answer is:
snafu
'chaos' is the definition.
(I've seen this before)
'upset united supporters' is the wordplay.
'upset' shows that the letters should be reversed in order.
'united' becomes 'u'.
'supporters' becomes 'fans' (fan is a kind of supporter).
'u'+'fans'='ufans'
'ufans' back-to-front is 'SNAFU'.
(Other definitions for snafu that I've seen before include "in a bad state" , "Chaotic state, mess" , "Chaos (US slang)" , "mess as usual" , "Mess table" .)