Getting on? That’s not about getting on (6)
I believe the answer is:
ageing
'getting on?' is the definition.
('age' can be a synonym of 'get on')
'not about getting on' is the wordplay.
'not' becomes 'e' (I can't explain this - if you can you should give a lot more credence to this answer).
'about' means one lot of letters goes inside another.
'getting on' becomes 'aging' (I've seen this before).
'e' going within 'aging' is 'AGEING'.
'that's' is the link.
(Other definitions for ageing that I've seen before include "Getting old" , "getting more advanced?" , "no longer fresh" , "Elderly" , "Declining" .)