Supporter about to enthuse briefly before United’s brilliant display (7)
I believe the answer is:
bravura
'brilliant display' is the definition.
(display of great talent)
'supporter about to enthuse briefly before united's' is the wordplay.
'supporter' becomes 'bra' (a bra gives support to breasts).
'about' indicates putting letters inside.
'to enthuse' becomes 'rave' (I've seen this in another clue).
'briefly' means to remove the last letter.
'before united' becomes 'u' (abbreviation. I am not sure about the 'before' bit.).
'rave' with its final letter removed is 'rav'.
'rav'+'u'='ravu'
'bra' placed around 'ravu' is 'BRAVURA'.
(Other definitions for bravura that I've seen before include "Brilliant and daring display" , "Display of boldness" , "Spirit and dash in music" , "Great skill and brilliance in performance" , "Virtuosity" .)