The crux of marrying is trust (4)
I believe the answer is:
gist
'the crux' is the definition.
(gist can mean the essence or crux of something)
'of marrying is trust' is the wordplay.
'of' indicates the answer is hidden within the clue.
'GIST' can be found hidden inside 'marrying is trust'.
(Other definitions for gist that I've seen before include "Key point" , "Essential idea" , "Main point of the matter" , "The pitch on the matter" , "Central idea" .)