Lives with son’s relative (3)
I believe the answer is:
sis
'relative' is the definition.
(short for sister)
'lives with son's' is the wordplay.
'lives' becomes 'is' ('be' can be a synonym of 'live').
'with' is a charade indicator (letters next to each other).
'is' after 's' is 'SIS'.
(Other definitions for sis that I've seen before include "Little sibling" , "Secret Intelligence Service (abbreviation)" , "Family member (informal)" , "Sibling (informal)" , "relative" .)