Not their (3)
I believe the answer is:
our
'not their' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for our that I've seen before include "group possession?" , "''Now is the winter of ... discontent'' (Richard III)" , "Possessed by us" , "Relating to us" , "'-- House' (Madness song)" .)