Otherwise, revolutionary’s backing art in a musical group (9)
I believe the answer is:
orchestra
'a musical group' is the definition.
(orchestra is a kind of musical group)
'otherwise revolutionary's backing art' is the wordplay.
'otherwise' becomes 'or'.
'revolutionary's' becomes 'che's' (Che Guevara the Marxist revolutionary).
'backing' says the letters should be written in reverse.
'art' reversed gives 'tra'.
'or'+'ches'+'tra'='ORCHESTRA'
'in' is the link.
(Other definitions for orchestra that I've seen before include "Eg, LSO, RPO" , "Large group of instrumentalists" , "Carthorse (anag)" , "Large group of musicians" , "Musicians, maybe in theatre" .)