Marriage, until now, is primarily working (5)
I believe the answer is:
union
'marriage' is the definition.
(synonyms)
'until now is primarily working' is the wordplay.
'primarily' indicates taking the first letters.
'working' becomes 'on' (on can mean working or activated).
The first letters of 'until now is' is 'uni'.
'uni'+'on'='UNION'
(Other definitions for union that I've seen before include "Merger like marriage" , "The USA in the USA" , "Association - guild" , "Coupling, merger" , "Combination" .)