Of course they'll discover it's owned by us (4)
I believe the answer is:
ours
'owned by us' is the definition.
(something which is ours belongs to us)
'of course they'll discover' is the wordplay.
'discover' says the answer is hidden in the clue (I've seen 'discovered' mean this (remove the letters covering the answer in the clue)).
'OURS' can be found hidden inside 'course theyll'.
'it's' is the link.
(Other definitions for ours that I've seen before include "British" , "The French bear" , "Yours and mine!" , "of the Queen?" , "Indiscriminate slaughter" .)