When couples do so, it's business as usual (7)
I believe the answer is:
cohabit
'when couples do so' is the definition.
I can't tell whether this defines the answer.
'business as usual' is the wordplay.
'business' becomes 'co' (short for company).
'as usual' becomes 'habit' (I can't explain this - if you can you should believe this answer much more).
'co'+'habit'='COHABIT'
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for cohabit that I've seen before include "Share a property" , "What some items do" , "16 ac 17 ac" , "Live together as unmarried couple" , "shack up" .)