Grind one's teeth (5)
I believe the answer is:
gnash
'grind one's teeth' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for gnash that I've seen before include "grate" , "Grind away" , "Grind (teeth) together" , "grinding action" , "Grind one's teeth" .)