Belonging to both of us (4)
I believe the answer is:
ours
'belonging to both of us' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ours that I've seen before include "What we share" , "of the Queen?" , "Relating or belonging to us" , "Belong to us both" , "The French bear" .)