Bring back supporter entering US - where it's a mess (5)
I believe the answer is:
snafu
'it's a mess' is the definition.
(I've seen this in another clue)
'bring back supporter entering us' is the wordplay.
'bring back' says the letters should be written in reverse.
'supporter' becomes 'fan' (fan is a kind of supporter).
'entering' means one lot of letters goes inside another.
'fan' put within 'us' is 'ufans'.
'ufans' backwards is 'SNAFU'.
'where' acts as a link.
(Other definitions for snafu that I've seen before include "Chaos (US slang)" , "Bloomer" , "Customary confusion" , "chaos, as usual" , "mess as usual" .)