Working musical boxes about to be sent back (6)
I believe the answer is:
active
'working' is the definition.
(similar in meaning)
'musical boxes about to be sent back' is the wordplay.
'musical' becomes 'evita' (Andrew Lloyd Webber musical).
'boxes' means one lot of letters goes inside another (the outside letters must 'box' inside letters in).
'about to be' becomes 'c' (I've seen this in other clues).
'sent back' says the letters should be written in reverse.
'evita' enclosing 'c' is 'evitca'.
'evitca' reversed gives 'ACTIVE'.
(Other definitions for active that I've seen before include "Moving frequently" , "Involved with sports" , "Working; moving" , "Mobile" , "erupting periodically" .)