Such surgeons often look down in the mouth (6)
I believe the answer is:
dental
'such' is the definition.
Although both the answer and definition are adjectives, I can't see how they can define each other.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for dental that I've seen before include "concerned with choppers" , "Concerning teeth" , "Concerned with teeth" , "sort of surgeon" , "Relating to teeth" .)