Harmony seldom initially found in marriage (6)
I believe the answer is:
unison
'harmony' is the definition.
(unison is a kind of harmony)
'seldom initially found in marriage' is the wordplay.
'initially' says to take the initial letters.
'found in' is an insertion indicator.
'marriage' becomes 'union' ('union' can be a synonym of 'marriage').
The initial letter of 'seldom' is 's'.
's' going within 'union' is 'UNISON'.
(Other definitions for unison that I've seen before include "with regard to the pitch" , "all players directed to the same pitch" , "Simultaneous utterance" , "Unscrupulous dealer" , "There's agreement" .)