Tough outer layer (4)
I believe the answer is:
rind
'tough outer layer' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for rind that I've seen before include "Cheese or fruit skin" , "Outer skin of citrus fruit" , "Zest" , "Hard outer skin, of an orange say" , "The peel of a fruit" .)