Practical joke is quietly offensive (5)
I believe the answer is:
prank
'practical joke' is the definition.
(I've seen this in another clue)
'quietly offensive' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'offensive' becomes 'rank' (similar in meaning).
'p'+'rank'='PRANK'
'is' is the link.
(Other definitions for prank that I've seen before include "Mischievous trick" , "Mischievous act done for fun and amusement" , "Practical joke or trick" , "(Amusing) trick" , "High-spirited mischief" .)