Nut from an oak (5)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">nut from an oak</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for acorn that I've seen before include "Source of a great oak" , "Model in stone" , "From which a tall tree may grow" , "mast" , "cup's content" .)