He takes part in an act of duplicity (8)
I believe the answer is:
duettist
'he' is the definition.
Both the definition and answer are singular nouns.
Perhaps you can see a link between them that I can't see?
'part in an act of duplicity' is the wordplay.
I cannot really see how this works, but
'part' could be 'ti' (ti is a kind of part) and 'ti' is found within the answer.
'act' could be 'duet' (duet is a kind of act) and 'duet' is found in the answer.
The remaining letters 'st' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'takes' acts as a link.
Can you help me to learn more?
(Other definitions for duettist that I've seen before include "One who plays or sings with another" , "Player with partner" .)