Quietly utter joke (5)
I believe the answer is:
prank
'joke' is the definition.
('prank' can be a synonym of 'joke')
'quietly utter' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'utter' becomes 'rank' ('rank' can be a synonym of 'utter').
'p'+'rank'='PRANK'
(Other definitions for prank that I've seen before include "Mischievous trick" , "Lark" , "High-spirited mischief" , "Mischievous act done for fun and amusement" , "(Amusing) trick" .)