A French individual working at marriage (5)
I believe the answer is:
union
'marriage' is the definition.
('union' can be a synonym of 'marriage')
'a french individual working' is the wordplay.
'a french' becomes 'un' ('a' in French).
'individual' becomes 'i'.
'working' becomes 'on'.
'un'+'i'+'on'='UNION'
'at' is the link.
(Other definitions for union that I've seen before include "workers' co-operative" , "Merger like marriage" , "political alliance" , "mating" , "Agreement -- wedlock" .)