Son lives as relative (3)
I believe the answer is:
sis
'relative' is the definition.
(short for sister)
'son lives' is the wordplay.
'son' becomes 's'.
'lives' becomes 'is' ('be' can be a synonym of 'live').
's'+'is'='SIS'
'as' is the link.
(Other definitions for sis that I've seen before include "relative" , "Little sibling" , "Sibling (informal)" , "Secret Intelligence Service (abbreviation)" , "spies ..." .)