Getting on, carrying on in unison (8)
I believe the answer is:
agreeing
'unison' is the definition.
(thesaurus)
'getting on carrying on' is the wordplay.
'getting on' becomes 'ageing' ('age' can be a synonym of 'get on').
'carrying' is an insertion indicator.
'on' becomes 're' ('on' can be mean 'with reference to').
'ageing' enclosing 're' is 'AGREEING'.
'in' is the link.
(Other definitions for agreeing that I've seen before include "Correspondent" , "OK" , "Settling on" , "Consenting" , "In accord" .)