Grind one's teeth (5)
I believe the answer is:
gnash
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">grind one's teeth</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for gnash that I've seen before include "grate" , "Grind away" , "Grind (teeth) together" , "grinding action" , "Grind one's teeth" .)