Concerned with a relative's motives (7)
I believe the answer is:
reasons
'motives' is the definition.
(reason is a kind of motive)
'concerned with a relative's' is the wordplay.
'concerned with' becomes 're'.
'relative's' becomes 'sons' (son is a kind of relative).
're'+'a'+'sons'='REASONS'
(Other definitions for reasons that I've seen before include "Ness oar (anag.)" , "Justifications or motives" , "Motives, rounds" , "Motives, explanations" , "Logically deduces" .)