It's yours and mine, of course (4)
I believe the answer is:
ours
'it's yours and mine' is the definition.
(I've seen this before)
'of course' is the wordplay.
'of' says the answer is hidden in the clue.
'OURS' is hidden within 'course'.
(Other definitions for ours that I've seen before include "of the Queen?" , "The French bear" , "Belong to us both" , "Relating or belonging to us" , "What we share" .)