Supporter: 'Five-nil? Well done!' (5)
I believe the answer is:
bravo
'well done' is the definition.
(I've seen this before)
'supporter five-nil?' is the wordplay.
'supporter' becomes 'bra' (I've seen this before).
'five' becomes 'v' ('V' can be a synonym of 'five').
'nil?' becomes 'o'.
'bra'+'v'+'o'='BRAVO'
(Other definitions for bravo that I've seen before include "A cry meaning well done" , "B in radio communications" , "How broadcaster might begin birthday" , "Cry of approval" , "next to alpha" .)