Son in marriage agreement (6)
I believe the answer is:
unison
'agreement' is the definition.
(unison is a kind of agreement)
'son in marriage' is the wordplay.
'son' becomes 's'.
'in' means one lot of letters goes inside another.
'marriage' becomes 'union' ('union' can be a synonym of 'marriage').
's' placed inside 'union' is 'UNISON'.
(Other definitions for unison that I've seen before include "Public service trade union" , "The concordant trade union?" , "Concord; a trade union" , "Trade union (where all agree?)" , "Musical agreement" .)