“Birds” were for a long time included among “Animals” (7)
I believe the answer is:
pigeons
'birds' is the definition.
(pigeon is a kind of bird)
'long time included among animals' is the wordplay.
'long time' becomes 'eon' (eon is a kind of long time).
'included among' indicates putting letters inside.
'animals' becomes 'pigs' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'eon' inserted within 'pigs' is 'PIGEONS'.
'were for a' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for pigeons that I've seen before include "Type of type or nose" , "Columbiform birds" , "traditional pie filling" , "Town birds" , "Spin ego around the birds" .)