A harmless piece of mischief for a bird (4)
I believe the answer is:
lark
I believe this clue is a double definition.
'a harmless piece of mischief' is the first definition.
(I've seen this before)
'bird' is the second definition.
(lark is a kind of bird)
'for a' acts as a link.
(Other definitions for lark that I've seen before include "One singing high" , "One gets up early" , "bit of mischief" , "Bird that sings on the wing" , "jolly occasion" .)