A bird engaged in some activity (2,2)
I believe the answer is:
at it
'engaged in some activity' is the definition.
'at it' can be an answer for 'engaged' (I've seen this in another clue). I'm not certain of the 'in some activity' bit.
'a bird' is the wordplay.
'bird' becomes 'tit' (tit is a kind of bird).
'a'+'tit'='AT IT'
Can you help me to learn more?
(Other definitions for at it that I've seen before include "Making love" , "working hard" , "Up to no good" , "Occupied (in crime?)" , "Busy" .)