Poison from caries treated with nitrogen (7)
I believe the answer is:
arsenic
'poison' is the definition.
(thesaurus)
'caries treated with nitrogen' is the wordplay.
'treated with' indicates anagramming the letters.
'nitrogen' becomes 'n' ('N' can be a synonym of 'nitrogen').
'caries' with its letters anagrammed with 'n' gives 'ARSENIC'.
'from' acts as a link.
(Other definitions for arsenic that I've seen before include "Brittle grey-green semi-metal, atomic number 33" , "Element No 33" , "Ratsbane" , "As briefly identified" , "Poison found in races" .)