Carbon is used for filling in gold tooth (7)
I believe the answer is:
incisor
'tooth' is the definition.
(incisor is a kind of tooth)
'carbon is used for filling in gold' is the wordplay.
'carbon' becomes 'C' (C is the chemical symbol for carbon).
'used for filling' is an insertion indicator.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'c'+'is'='cis'
'in'+'or'='inor'
'cis' inserted inside 'inor' is 'INCISOR'.
(Other definitions for incisor that I've seen before include "companion could be Goofy" , "Chisel-edged tooth" , "Canine" , "cutter" , "Front tooth" .)