To perform subtly, felt maybe piano should intervene (9)
I believe the answer is:
underplay
'to perform subtly' is the definition.
(to play or perform in a subtle way)
'felt maybe piano should intervene' is the wordplay.
'felt maybe' becomes 'underlay' (felt laid under a carpet).
'piano' becomes 'p'.
'should intervene' means one lot of letters goes inside another.
'underlay' enclosing 'p' is 'UNDERPLAY'.
(Other definitions for underplay that I've seen before include "show restraint" , "having to act with restraint" , "Perform (a role) in a restrained way" , "Reduce stress on" , "Fail to stress" .)