Belonging to the thing (3)
I believe the answer is:
its
'belonging to the thing' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for its that I've seen before include "Sit (anag.)" , "Belonging to something nonhuman" , "The thing's" , "Neuter possessive pronoun" , "Belonging to a particular thing" .)