Doing well — as birds may be? (6)
I believe the answer is:
flying
I believe this is a double definition.
'doing well' is the first definition.
(someone who is flying is doing well at a task)
'as birds may be?' is the second definition.
(Other definitions for flying that I've seen before include "Travelling by plane" , "Moving quickly" , "-- Squad; -- doctor" , "High speed steam engine (1923)" , "Going by air" .)