Couple’s investment (4)
I believe the answer is:
bond
I believe this clue is a double definition.
'couple's' is the first definition.
(thesaurus)
'investment' is the second definition.
(bond is a type of investment)
(Other definitions for bond that I've seen before include "Spy - stick" , "Fellow feeling" , "Join; secret agent" , "James ...., fiction's 007" , "secure?" .)