Onset of caries is found in one new gold tooth (7)
I believe the answer is:
incisor
'tooth' is the definition.
(I know that incisor is a type of tooth)
'onset of caries is found in one new gold' is the wordplay.
'onset of' indicates taking the first letters.
'found in' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'gold' becomes 'or' ('or' is the heraldic name for gold).
The first letter of 'caries' is 'c'.
'c'+'is'='cis'
'i'+'n'+'or'='inor'
'cis' placed within 'inor' is 'INCISOR'.
(Other definitions for incisor that I've seen before include "Type of tooth" , "carnivore requires?" , "Chisel-edged tooth" , "Canine" , "Narrow-edged tooth" .)