Garfield's successor published an impressive author (6,7)
I believe the answer is:
arthur ransome
'author' is the definition.
'garfield's successor published an impressive' is the wordplay.
'garfield's successor' becomes 'arthur' (I've seen this in another clue).
'published' becomes 'ran' (running is a kind of publishing).
'an impressive' becomes 'some' (I can't justify this - if you can you should believe this answer much more).
'arthur'+'ran'+'some'='ARTHUR RANSOME'
Can you help me to learn more?
(Other definitions for arthur ransome that I've seen before include "Swallows and Amazons author" , "Children's author" .)