Relative index primarily used in adjustment of NSW loan (3-2-3)
I believe the answer is:
son-in-law
'relative' is the definition.
(son-in-law is a kind of relative)
'index primarily used in adjustment of nsw loan' is the wordplay.
'primarily used' suggests taking the first letters.
'in' is an insertion indicator.
'adjustment of' is an anagram indicator (I've seen this in other clues).
The initial letter of 'index' is 'i'.
'nsw'+'loan'='nswloan'
'nswloan' is an anagram of 'sonnlaw'.
'i' put within 'sonnlaw' is 'SON-IN-LAW'.
(Other definitions for son-in-law that I've seen before include "Daughter's husband" , "Relative by marriage" , "He's married to your daughter" , "Member of extended family" .)