Good novel, their worst author (11)
I believe the answer is:
ghostwriter
'author' is the definition.
(ghostwriter is a kind of author)
'good novel their worst' is the wordplay.
'good' becomes 'g' (abbreviation).
'novel' is an anagram indicator (letters in a new or novel order).
'their'+'worst'='theirworst'
'theirworst' anagrammed gives 'hostwriter'.
'g'+'hostwriter'='GHOSTWRITER'
(Other definitions for ghostwriter that I've seen before include "scibe for the incapable" , "Author denied credit" , "second involved in an autobiography?" , "Maybe Sheridan Le Fanu" .)