They don't really look down in the mouth! (8)
I believe the answer is:
eyeteeth
'they don't really' is the definition.
The definition suggests an adverb but the answer is not.
'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 believe this answer much more).
'eye'+'teeth'='EYETEETH'
Can you help me to learn more?
(Other definitions for eyeteeth that I've seen before include "Organs of sight in the mouth?" , "Canine bones in upper jaw - visualise them" , "Four pointed structures in the mouth" , "Canine bones in upper jaw, under the optic" , "choppers" .)