Writer achieving gold with mythical character (6)
I believe the answer is:
'writer' is the definition.
'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).
'achieving' is the link.
(Other definitions for author that I've seen before include "Creative artist", "Dramatist", "Generator of written work", "Ford, for example", "Write".)