Flier's mischievous activity (4)
I believe the answer is:
lark
This clue is a double definition.
'flier's' is the first definition.
(lark is a kind of birdanimal)
'mischievous activity' is the second definition.
(I've seen this before)
(Other definitions for lark that I've seen before include "Harmless adventure" , "jolly occasion" , "Songbird - bit of fun" , "Early bird with morning song" , "bit of mischief" .)