Sanity is concerning a family member (6)
I believe the answer is:
reason
'sanity' is the definition.
(reason is a kind of sanity)
'concerning a family member' is the wordplay.
'concerning' becomes 're'.
'family member' becomes 'son' (I've seen this in other clues).
're'+'a'+'son'='REASON'
'is' is the link.
(Other definitions for reason that I've seen before include "Purpose" , "logical argument" , "Explanation or motive" , "The right exercise of the mind" , "Sanity and logic" .)