Whetstone (4)
I believe the answer is:
hone
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">whetstone</span>' is the definition.<br/ ><span class="explanation-format__explanation">(hone is a kind of whetstone)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for hone that I've seen before include "Polish" , "Smooth stone" , "Sharpen tool on stone" , "Sharpen (a blade)" , "Strop" .)