Start of sensuousness in marriage harmony (6)
I believe the answer is:
unison
'harmony' is the definition.
(unison is a kind of harmony)
'start of sensuousness in marriage' is the wordplay.
'start of' suggests taking the first letters.
'in' is an insertion indicator.
'marriage' becomes 'union' (synonyms).
The initial letter of 'sensuousness' is 's'.
's' placed into 'union' is 'UNISON'.
(Other definitions for unison that I've seen before include "concert" , "using same pitch" , "Simultaneous utterance" , "all players directed to the same pitch" , "Unanimity" .)