A bird busy as a bee? (2,2)
I believe the answer is:
at it
'busy as a bee?' is the definition.
(I've seen this in another clue)
'a bird' is the wordplay.
'bird' becomes 'tit' (tit is a kind of bird).
'a'+'tit'='AT IT'
(Other definitions for at it that I've seen before include "occupied in a particular way" , "Busy" , "Engaged in something particular" , "Up to no good" , "engaged in dubious activity?" .)