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