A writer succeeded, welcoming all the Muses in range (9)
I believe the answer is:
'range' is the definition.
(I know that Apennines is a range)
'a writer succeeded welcoming all the muses' is the wordplay.
'writer' becomes 'pen' (something which writes).
'succeeded' becomes 's' (genealogical abbreviation).
'welcoming' means one lot of letters goes inside another (in sense of 'inviting in').
'all the muses' becomes 'nine' (the nine muses of Greek mythology).
'pens' enclosing 'nine' is 'pennines'.
'in' acts as a link.
(Other definitions for apennines that I've seen before include "Italian mountain chain" , "Italian mountains" , "Mountain range in central Italy" , "European range" , "Great mountain range in Italy (9)" .)