Complete agreement sees son in marriage (6)
I believe the answer is:
unison
'complete agreement' is the definition.
(I've seen this before)
'son in marriage' is the wordplay.
'son' becomes 's'.
'in' indicates putting letters inside.
'marriage' becomes 'union' ('union' can be a synonym of 'marriage').
's' put inside 'union' is 'UNISON'.
'sees' is the link.
(Other definitions for unison that I've seen before include "Trade union (where all agree?)" , "As one voice" , "State of corresponding exactly; occurring together" , "using same pitch" , "Sameness in pitch" .)