Plain heartless in extremes to enact instrument (8)
I believe the answer is:
clarinet
'instrument' is the definition.
(I know that clarinet is a clarinet)
'plain heartless in extremes to enact' is the wordplay.
I cannot really understand how this works, but
'in' is present in the answer.
The remaining letters 'claret' 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 clarinet that I've seen before include "Woodwind instrument - Artie Shaw played one" , "musician" , "Woodwind instrument played by Artie Shaw and Benny Goodman" , "Blow that!" , "Musical wind instrument as played by Benny Goodman" .)