One I scorn breaking tooth (7)
I believe the answer is:
incisor
'tooth' is the definition.
(I know that incisor is a type of tooth)
'one i scorn breaking' is the wordplay.
'one' becomes 'I' (Roman numeral).
'breaking' is an anagram indicator.
'i'+'scorn'='iscorn'
'iscorn' anagrammed gives 'ncisor'.
'i'+'ncisor'='INCISOR'
(Other definitions for incisor that I've seen before include "One takes a bite" , "oral equipment" , "carnivore requires?" , "something with bite" , "Ivory perhaps" .)