Two compositions for which payment is determined by amount produced (9)
I believe the answer is:
piecework
'for which payment is determined by amount produced' is the definition.
The definition suggests a singular noun which matches the answer.
'two compositions' is the wordplay.
'two compositions' means two replacements for 'composition'.
'composition' becomes 'piece' ('piece' can be a synonym of 'composition').
'composition' becomes 'work' (I can't explain this - if you can you should believe this answer much more).
'piece'+'work'='PIECEWORK'
Can you help me to learn more?
(Other definitions for piecework that I've seen before include "Payment by results" , "doing jigsaw?" , "Method of payment" .)