Hide singular relationship (4)
I believe the answer is:
skin
'hide' is the definition.
(a hide is an animal skin)
'singular relationship' is the wordplay.
'singular' becomes 's' (abbreviation in grammar).
'relationship' becomes 'kin' (I've seen this in other clues).
's'+'kin'='SKIN'
(Other definitions for skin that I've seen before include "Living tissue" , "Surface layer" , "Outer layer of tissue" , "Fell" , "Thin outer layer" .)