Marriage of our Jack? (5)
I believe the answer is:
union
This clue is a double definition.
'marriage' is the first definition.
(synonyms)
'our jack?' is the second definition.
(the union jack flag)
'of' acts as a link.
(Other definitions for union that I've seen before include "Organisation of employees for bargaining purposes" , "The coming together of separate parts" , "Trade organisation to represent workers" , "Workers' group" , "Workers' organisation or marriage" .)