Ancient fellow in the police force (3,4)
I believe the answer is:
old bill
'the police force' is the definition.
(slang term)
'ancient fellow' is the wordplay.
'ancient' becomes 'old' (similar in meaning).
'fellow' becomes 'bill' (I have seen 'The fellow' mean 'bill' so perhaps 'fellow' could also mean 'bill').
'old'+'bill'='OLD BILL'
'in' is the link.
(Other definitions for old bill that I've seen before include "forgotten account" , "The police in the UK" , "previous charges" , "Police (slang)" , "Policeman" .)