Acorn tree (3)
I believe the answer is:
oak
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">acorn tree</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 oak that I've seen before include "Acorn-bearing tree" , "Acorn tree" , "door at Oxford" , "Common forest tree" , "The King of the Forest" .)