Do they look down in the mouth? (8)
I believe the answer is:
eyeteeth
'do they' is the definition.
The definition suggests a plural noun which matches the answer.
'look down in the mouth?' is the wordplay.
'look' becomes 'eye' (eyeing is a kind of looking).
'down in the mouth?' becomes 'teeth' (I can't explain this - if you can you should give a lot more credence to this answer).
'eye'+'teeth'='EYETEETH'
Can you help me to learn more?
(Other definitions for eyeteeth that I've seen before include "Canines in upper jaw, under the optics" , "they're sharp" , "Canine bones in upper jaw - visualise them" , "Organs of sight in the mouth?" , "Four pointed structures in the mouth" .)