Argue about a family member (6)
I believe the answer is:
reason
'argue' is the definition.
(synonyms)
'about a family member' is the wordplay.
'about' becomes 'rea' (I've seen this in other clues).
'a family member' becomes 'son' (I've seen this in another clue).
'rea'+'son'='REASON'
(Other definitions for reason that I've seen before include "Motive for an action" , "Incentive" , "Justification or motive" , "'Motivation, grounds (6)'" , "Sanity and logic" .)