Busy contralto in revolutionary musical (6)
I believe the answer is:
active
'busy' is the definition.
(similar in meaning)
'contralto in revolutionary musical' is the wordplay.
'contralto' becomes 'c' (musical abbreviation).
'in' is an insertion indicator.
'revolutionary' shows that the letters should be reversed in order (letters revolve around).
'musical' becomes 'evita' (Andrew Lloyd Webber musical).
'evita' written backwards gives 'ative'.
'c' going into 'ative' is 'ACTIVE'.
(Other definitions for active that I've seen before include "On the go" , "Keeping busy or operative" , "Moving, doing things" , "Working; moving" , "Dynamic, functioning" .)