Writer achieving gold with mythical character (6)
I believe the answer is:
author
'writer' is the definition.
(synonyms)
'gold with mythical character' is the wordplay.
'gold' becomes 'Au' (Au is the chemical symbol for gold).
'with' is a charade indicator (letters next to each other).
'mythical character' becomes 'Thor' (Norse god).
'au'+'thor'='AUTHOR'
'achieving' is the link.
(Other definitions for author that I've seen before include "Writer of a book, say" , "Book producer" , "Creator of written work" , "Producer of written work" , "Writer of a book, article or document" .)