Skin of sweetheart covered in froth (7)
I believe the answer is:
leather
'skin' is the definition.
(leather is a kind of skin)
'sweetheart covered in froth' is the wordplay.
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'covered in' is an insertion indicator.
'froth' becomes 'lather' (lather is a kind of froth).
'e' placed inside 'lather' is 'LEATHER'.
'of' is the link.
(Other definitions for leather that I've seen before include "Flog; hide" , "Polishing cloth" , "kid, say" , "Material made from animal skin" , "Animal skins" .)