Surface layer (4)
I believe the answer is:
skin
'surface layer' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for skin that I've seen before include "Hide - flay" , "Scrape, as the knee" , "Outer layer of a fruit or vegetable" , "Outer layer of, eg, apple" , "Living tissue" .)