Business custom is to share accommodation (7)
I believe the answer is:
cohabit
'to share accommodation' is the definition.
(I've seen this before)
'business custom' is the wordplay.
'business' becomes 'co'.
'custom' becomes 'habit' (habit is a kind of custom).
'co'+'habit'='COHABIT'
'is' is the link.
(Other definitions for cohabit that I've seen before include "Share residence" , "House-share" , "Share accommodation" , "Live together as unmarried couple" , "Live together as husband and wife" .)