Tree's protective layer (4)
I believe the answer is:
bark
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">tree's protective layer</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for bark that I've seen before include "coverage of the Oaks?" , "Outer part of tree" , "old vessel" , "Setter's sound" , "Bay" .)