Motive with respect to a family member (6)
I believe the answer is:
reason
'motive' is the definition.
(reason is a kind of motive)
'with respect to a family member' is the wordplay.
'with respect to' becomes 're' ('re' means regarding or about).
'family member' becomes 'son' (I've seen this in other clues).
're'+'a'+'son'='REASON'
(Other definitions for reason that I've seen before include "Logic, sense" , "Justification needed" , "use of wit" , "Purpose" , "Motive; argue" .)