Animal needing a long time to become a proficient flyer (6)
I believe the answer is:
pigeon
'a proficient flyer' is the definition.
(bird known for flying long distances)
'animal needing a long time' is the wordplay.
'animal' becomes 'pig' (I've seen this in other clues).
'needing' says to put letters next to each other.
'a long time' becomes 'eon' (eon is a kind of long time).
'pig'+'eon'='PIGEON'
'to become' is the link.
(Other definitions for pigeon that I've seen before include "Common city bird" , "Common town bird" , "a visitor to Trafalgar Square" , "Bird (with a hole?)" , "Bird of towns and wood" .)