Backing musical about 3 worker (9)
I believe the answer is:
operative
'worker' is the definition.
(I've seen this before)
'backing musical about 3' is the wordplay.
'backing' shows that the letters should be reversed in order.
'musical' becomes 'evita' (Andrew Lloyd Webber musical).
'about p' becomes 'repo' (I can't justify this - if you can you should believe this answer much more).
'evita'+'repo'='evitarepo'
'evitarepo' in reverse letter order is 'OPERATIVE'.
Can you help me to learn more?
(Other definitions for operative that I've seen before include "Having force - skilled worker" , "Efficient" , "Working (person)" , "Significant" , "secret agent" .)