Passenger pigeon or dodo? Certainly no spring chicken! (3,4)
I believe the answer is:
old bird
'certainly no spring chicken' is the definition.
The definition suggests a singular noun which matches the answer.
'passenger pigeon or dodo?' is the wordplay.
I cannot quite see how this works, but
'dodo?' could be 'bird' (dodo is a kind of bird) and 'bird' is found in the answer.
The remaining letters 'old' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for old bird that I've seen is " Astute and experienced person".)