Worker’s musical production originally viewed in formal wear (9)
I believe the answer is:
operative
'worker's' is the definition.
(I've seen this before)
'musical production originally viewed in formal wear' is the wordplay.
'musical production' becomes 'opera' (I've seen this in another clue).
'originally' suggests taking the first letters.
'in' indicates putting letters inside.
'formal wear' becomes 'tie' (formal item of clothing).
The first letter of 'viewed' is 'v'.
'v' inserted inside 'tie' is 'tive'.
'opera'+'tive'='OPERATIVE'
(Other definitions for operative that I've seen before include "Working (person)" , "Worker in manufacturing" , "Having force - skilled worker" , "American detective" , "Efficient" .)