Something to do with teeth (5)
I believe the answer is:
gnash
'teeth' is the definition.
The definition and answer can be both related to bodily activities as well as being verbs in their base form.
Maybe you can see an association between them that I don't see?
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for gnash that I've seen before include "Grind (teeth) together" , "Grind one's teeth" , "Hangs (anag.)" , "Strike one's teeth together in rage" , "Grind away" .)