Naive admission of older age (8)
I believe the answer is:
immature
'naive' is the definition.
(similar in meaning)
'admission of older age' is the wordplay.
I cannot quite see how this works, but
'age' could be 'mature' (synonyms) and 'mature' is present in the answer.
The remaining letters 'im' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for immature that I've seen before include "Not yet fully-developed" , "Not fully ripe or developed" , "Undeveloped - unripe" , "Not yet fully grown or developed" , "Underdeveloped - childish" .)