Metal repeatedly used by cartoon hero (6)
I believe the answer is:
tintin
'cartoon hero' is the definition.
(I've seen this before)
'metal repeatedly used by' is the wordplay.
'metal repeatedly' can mean two substitutions for 'metal'.
'metal' becomes 'tin' (tin is a kind of metal).
'metal' becomes 'tin' (tin is a kind of metal).
'used by' says to put letters next to each other.
'tin'+'tin'='TINTIN'
(Other definitions for tintin that I've seen before include "Herge's adventurer" , "Herge's adventurous young Belgian reporter" , "Comic character" , "Herge's cartoon hero" , "cartoon character" .)