Oak nut (5)
I believe the answer is:
acorn
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">oak nut</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for acorn that I've seen before include "oak's fruit" , "Tree seed" , "mast" , "From which a "tall oak" may grow" , "old computer?" .)