Couple's thing (4)
I believe the answer is:
item
I believe this is a double definition.
'couple's' is the first definition.
(a romantic couple are 'an item')
'thing' is the second definition.
(an item is a thing)
(Other definitions for item that I've seen before include "Individual object" , "One thing among many" , "Two in a romantic relationship?" , "Entry (in an account)" , "pair?" .)