Backing group of Verdi's outrivalled top players (8)
I believe the answer is:
'top players' is the definition.
(I've seen this before)
'backing group of verdi's outrivalled' is the wordplay.
'backing' says the letters should be written in reverse.
'group of' indicates a hidden word (group of letters from the clue).
'isoutriv' can be found hidden inside 'verdis outrivalled'.
'isoutriv' back-to-front is 'VIRTUOSI'.
(Other definitions for virtuosi that I've seen before include "Skilled musicians" , "Top players" .)