Fruit of the oak (5)
I believe the answer is:
acorn
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fruit of the 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 "Potentially future tree" , "Dirt on a liquid surface" , "oak's fruit" , "From which a tall tree may grow" , "old computer?" .)