Live together, it's a firm tradition (7)
I believe the answer is:
cohabit
'live together' is the definition.
(synonyms)
'a firm tradition' is the wordplay.
'a firm' becomes 'co' (abbreviation for company).
'tradition' becomes 'habit' (habit is a kind of tradition).
'co'+'habit'='COHABIT'
'it's' acts as a link.
(Other definitions for cohabit that I've seen before include "Share residence" , "What some items do" , "Live together, married or not" , "What to do with partner" , "Share a property" .)