Belonging to that thing (3)
I believe the answer is:
its
'belonging to that thing' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for its that I've seen before include "The thing's" , "Belonging to a particular thing" , "Another form of 'tis" , "Belonging to something nonhuman" , "Sit (anag.)" .)