Son and relations hide (4)
I believe the answer is:
skin
'hide' is the definition.
('skin' can be a synonym of 'hide')
'son and relations' is the wordplay.
'son' becomes 's'.
'and' says to put letters next to each other.
'relations' becomes 'kin' (I've seen this before**).
's'+'kin'='SKIN'
(Other definitions for skin that I've seen before include "Outer layer of tissue" , "Outer layer of a fruit or vegetable" , "Outer surface" , "Peel; drum head" , "Hide - flay" .)