Final word on one couple's facilities (9)
I believe the answer is:
amenities
'facilities' is the definition.
(I have seen 'Useful facilities ' mean 'amenities' so perhaps 'facilities' could also mean 'amenities')
'final word on one couple's' is the wordplay.
'final word' becomes 'amen' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'one' becomes 'i' (roman numeral).
'couple's' becomes 'ties' (both can mean to join).
'amen'+'i'+'ties'='AMENITIES'
(Other definitions for amenities that I've seen before include "pleasing features" , "Useful facilities" , "Equipment" , "conveniences" , "useful services?" .)