That belonging to us may turn sour (4)
I believe the answer is:
ours
'that belonging to us' is the definition.
(I've seen this before)
'turn sour' is the wordplay.
'turn' is an anagram indicator.
'sour' anagrammed gives 'OURS'.
'may' is the link.
(Other definitions for ours that I've seen before include "of the Queen?" , "Relating or belonging to us" , "Indiscriminate slaughter" , "What we share" , "Yours and mine!" .)