Assuming to be no longer young (7,2)
I believe the answer is:
getting on
'no longer young' is the definition.
(I've seen this before)
'assuming to be' is the wordplay.
I cannot quite see how this works, but
'be' could be 'go' (going is a kind of being) and 'go' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for getting on that I've seen before include "Like group of friends" , "doing well" , "showing signs of age?" , "Succeeding as a boarder" , "no longer young" .)