Tough outer layer (4)
I believe the answer is:
rind
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">tough outer layer</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)