The main point of connubial relations (3)
I believe the answer is:
nub
'the main point' is the definition.
(I've seen this before)
'of connubial relations' is the wordplay.
'of' indicates a hidden word.
'NUB' is hidden within 'connubial relations'.
(Other definitions for nub that I've seen before include "Metal piece" , "'Gist, central idea (3)'" , "Small lump - main point" , "Crux; lump" , "Central point of bun" .)