Oak tree fruit (5)
I believe the answer is:
acorn
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">oak tree 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 "Potential oak" , "cup's content" , "From which a mighty oak may grow?" , "Source of a great oak" , "fruit in a cup" .)