Attire dishevelled by supporter upset referee (9)
I believe the answer is:
arbitrate
'referee' is the definition.
(thesaurus)
'attire dishevelled by supporter upset' is the wordplay.
'dishevelled' indicates an anagram.
'by' says to put letters next to each other.
'supporter' becomes 'bra' (I've seen this before).
'upset' shows that the letters should be reversed in order.
'bra' in reverse letter order is 'arb'.
'attire' with letters rearranged gives 'itrate'.
'itrate' after 'arb' is 'ARBITRATE'.
(Other definitions for arbitrate that I've seen before include "Act between disputing parties" , "Adjudge" , "judge" , "Help settle a dispute" , "Decide" .)