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