Grind, as teeth (5)
I believe the answer is:
'grind as 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 "Strike one's teeth together in rage" , "grinding action" , "Grind (teeth) together" , "Grind one's teeth" , "Hangs (anag.)" .)