Firm pay without a duplicate (4)
I believe the answer is:
copy
'duplicate' is the definition.
(thesaurus)
'firm pay without a' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'without' suggests deleting specific letters.
'pay' with 'a' taken out is 'py'.
'co'+'py'='COPY'
(Other definitions for copy that I've seen before include "Matter to be printed; ape" , "Plagiarise, material for newspaper" , "stuff to be published" , "Imitation or reproduction" , "Replica; text for printing" .)