Acting on the side (2-4)
I believe the answer is:
by-play
'acting' is the definition.
Both the answer and definition are singular nouns.
Perhaps there's an association between them I don't understand?
'the side' is the wordplay.
I cannot really understand how this works, but
'side' could be 'by' (synonyms) and 'by' is present in the answer.
The remaining letters 'play' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'on' acts as a link.
Can you help me to learn more?
(Other definitions for by-play that I've seen before include "Stage business" , "Secondary action, often humorous" , "Secondary action on stage" , "Not the main action" , "Side action on stage" .)