United supporters turning up in a mess (5)
I believe the answer is:
snafu
'in a mess' is the definition.
(thesaurus)
'united supporters turning up' is the wordplay.
'united' becomes 'u'.
'supporters' becomes 'fans' (fan is a kind of supporter).
'turning up' says the letters should be written backwards.
'u'+'fans'='ufans'
'ufans' back-to-front is 'SNAFU'.
(Other definitions for snafu that I've seen before include "Customary confusion" , "Bloomer" , "Slangy acronym meaning in total confusion" , "in a bad state" , "Chaotic state, mess" .)