Make smooth (4)
I believe the answer is:
sand
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">make smooth</span>' is the definition.<br/ ><span class="explanation-format__explanation">(sanding is a kind of making smooth)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for sand that I've seen before include "Yellow-brown colour" , "Fine grit" , "Very tiny grains of rock" , "woman writer Chopin loved" , "Result of rock erosion" .)