Little mischief? One’s played quietly (3)
I believe the answer is:
imp
'little mischief?' is the definition.
(mischievous person)
'one's played quietly' is the wordplay.
'one's' becomes 'im' ('one is' is a formal way of saying 'I am').
'played quietly' becomes 'p' (musical abbreviation for 'piano' - played softly).
'im'+'p'='IMP'
(Other definitions for imp that I've seen before include "Little troublemaker" , "Mischievous creature" , "Small devil or sprite" , "old-fashioned graft" , "Small mischievous spirit" .)