Animals, over a long period of time, becoming winged creatures (7)
I believe the answer is:
pigeons
'animals' is the definition.
(pigeon is a kind of animal)
'over a long period of time becoming winged creatures' is the wordplay.
'over' is an insertion indicator.
'a long period of time' becomes 'eon' (I've seen this in another clue).
'becoming winged creatures' becomes 'pigs' (I can't justify this - if you can you should believe this answer much more).
'eon' inserted within 'pigs' is 'PIGEONS'.
Can you help me to learn more?
(Other definitions for pigeons that I've seen before include "Town birds" , "Cooing birds" , "Columbiform birds" , "Spin ego around the birds" , "traditional pie filling" .)