Someone who writes in French, to the Thunderer? (6)
I believe the answer is:
author
'someone who writes' is the definition.
'in french to the thunderer?' is the wordplay.
'in french to' becomes 'au' ('au' in French).
'the thunderer?' becomes 'thor' (Norse thunder god).
'au'+'thor'='AUTHOR'
(Other definitions for author that I've seen before include "Writer of a book, say" , "Professional writer" , "Writer of a book or play" , "Producer of written work" , "Book producer" .)