Inheritors pursuing terrain primarily belonging to them (6)
I believe the answer is:
theirs
'belonging to them' is the definition.
(I've seen this before)
'inheritors pursuing terrain primarily' is the wordplay.
'inheritors' becomes 'heirs' ('heir' can be a synonym of 'inheritor').
'pursuing' means one lot of letters go next to another.
'primarily' says to take the initial letters.
The initial letter of 'terrain' is 't'.
'heirs' after 't' is 'THEIRS'.
(Other definitions for theirs that I've seen before include "Belonging to those people" , "Owned by them" .)