Such teeth stick out! (Also in) (6)
I believe the answer is:
canine
'such teeth stick' is the definition.
The answer and definition can be both body parts as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'out also in' is the wordplay.
'out' is an insertion indicator.
'also' becomes 'cane' (I can't explain this - if you can you should give a lot more credence to this answer).
'cane' placed around 'in' is 'CANINE'.
Can you help me to learn more?
(Other definitions for canine that I've seen before include "such as Lassie" , "Fang" , "A tooth; of our best friend" , "Pointed conical tooth" , "Relating to dogs" .)