To live together is a firm custom (7)
I believe the answer is:
cohabit
'to live together' is the definition.
('cohabit' can be a synonym of 'live together')
'firm custom' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'custom' becomes 'habit' (habit is a kind of custom).
'co'+'habit'='COHABIT'
'is a' acts as a link.
(Other definitions for cohabit that I've seen before include "Live together, married or not" , "Share residence" , "Share a property" , "House-share" , "What to do with partner" .)