Accepting second marriage? Together, we're in this (6)
I believe the answer is:
unison
'together we're in this' is the definition.
The definition suggests a singular noun which matches the answer.
'accepting second marriage?' is the wordplay.
'accepting' means one lot of letters goes inside another (inserted letters are accepted).
'second' becomes 's' (common abbreviation).
'marriage?' becomes 'union' ('union' can be a synonym of 'marriage').
's' inserted into 'union' is 'UNISON'.
Can you help me to learn more?
(Other definitions for unison that I've seen before include "Total agreement or concord" , "Voices in complete agreement" , "Public service trade union" , "Simultaneous action" , "Accord" .)