Accept, we're told, 50 in emolument for subsidiary action (2-4)
I believe the answer is:
'action' is the definition.
(I have seen 'Side action' mean 'by-play' so perhaps 'action' could also mean 'by-play')
'accept we're told 50 in emolument for subsidiary' is the wordplay.
I cannot really understand how this works, but
'50' could be 'l' (Roman numeral) and 'l' is found within the answer.
'emolument' could be 'pay' (I've seen this in another clue) and 'pay' is present in the remaining letters.
The remaining letters 'by' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for by-play that I've seen before include "Stage business" , "Side action on stage" , "Secondary action on stage" , "Secondary action, often humorous" , "Not the main action" .)