First of three successors belonging to them (6)
I believe the answer is:
theirs
'belonging to them' is the definition.
(I've seen this before)
'first of three successors' is the wordplay.
'first of' suggests taking the first letters.
'successors' becomes 'heirs' ('heir' can be a synonym of 'successor').
The first letter of 'three' is 't'.
't'+'heirs'='THEIRS'
(Other definitions for theirs that I've seen before include "Owned by them" , "Belonging to those people" .)