Back a theatre group initially over stage work (5)
I believe the answer is:
opera
'stage work' is the definition.
(I've seen this before)
'back a theatre group initially over' is the wordplay.
'back' says the letters should be written backwards.
'theatre group' becomes 'rep' (abbreviation for repertory theatre).
'initially' suggests taking the first letters.
The first letter of 'over' is 'o'.
'a'+'rep'+'o'='arepo'
'arepo' backwards is 'OPERA'.
(Other definitions for opera that I've seen before include ""Rigoletto", for example" , "eg Peter Grimes" , "eg 'Aida'" , "La Boheme or Rigoletto" , "Drama with singing" .)