Stage work the writer's penning finally about worker (9)
I believe the answer is:
operative
'worker' is the definition.
(I know that operative is a type of detective)
'stage work the writer's penning finally about' is the wordplay.
'stage work' becomes 'opera' (I've seen this before).
'the writer's' becomes 'ive' (the writer of the crossword).
'penning' means one lot of letters goes inside another.
'finally' suggests the final letters.
The last letter of 'about' is 't'.
'opera'+'ive'='operaive'
'operaive' placed around 't' is 'OPERATIVE'.
(Other definitions for operative that I've seen before include "American detective" , "secret agent" , "Worker in manufacturing" , "desired effect" , "Efficient" .)