Age of bird, not beast (3)
I believe the answer is:
eon
'age' is the definition.
(I know that eon is a type of age)
'bird not beast' is the wordplay.
I cannot quite see how this works, but
'not' could be 'o' (I've seen this in other clues) and 'o' is found in the answer.
The remaining letters 'en' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'of' acts as a link.
Can you help me to learn more?
(Other definitions for eon that I've seen before include "Huge time period" , "In time, long division" , "Major division of geological time" , "Age, epoch" , "Era" .)