Fret about VAR upsetting a brilliant display (7)
I believe the answer is:
bravura
'brilliant display' is the definition.
(display of great talent)
'fret about var upsetting a' is the wordplay.
'fret' becomes 'rub' (synonyms).
'about' means one lot of letters goes inside another.
'upsetting' is a reversal indicator.
'rub' going around 'var' is 'ruvarb'.
'ruvarb' backwards is 'bravur'.
'bravur'+'a'='BRAVURA'
(Other definitions for bravura that I've seen before include "Virtuosity" , "Display of boldness" , "Brilliant and daring display" , "Display of great skill" , "Brilliance - dash" .)