Argue concerning a member of the family (6)
I believe the answer is:
reason
'argue' is the definition.
('reason' can be a synonym of 'argue')
'concerning a member of the family' is the wordplay.
'concerning' becomes 're'.
'member of the family' becomes 'son' (I've seen this in other clues).
're'+'a'+'son'='REASON'
(Other definitions for reason that I've seen before include "use of wit" , "Cause; think logically" , "Logic, sense" , "Logic; excuse" , "brain operations" .)