Writer's assurance is backed by forger (9)
I believe the answer is:
wordsmith
'writer's' is the definition.
(wordsmith is a kind of writer)
'assurance is backed by forger' is the wordplay.
'assurance' becomes 'word' (I've seen this in another clue).
'is backed by' is a charade indicator (letters next to each other).
'forger' becomes 'smith' (I've seen this before).
'word'+'smith'='WORDSMITH'
(Other definitions for wordsmith that I've seen before include "Skilled writer or orator" , "Person skilled at writing" , "he'd be good at coining?" , "fluent writer" , "Author" .)