Influence exists through established writer (8)
I believe the answer is:
essayist
'writer' is the definition.
(essayist is a kind of writer)
'influence exists through established' is the wordplay.
'influence' becomes 'say' (I've seen this in other clues).
'exists' becomes 'is' ('be' can be a synonym of 'exist').
'through' indicates putting letters inside (I've seen this in other clues).
'established' becomes 'est' (I've seen this in other clues).
'say'+'is'='sayis'
'sayis' going inside 'est' is 'ESSAYIST'.
(Other definitions for essayist that I've seen before include "kind of writer" , "Writer (e.g. Lamb)" , "Montaigne, say" , "Writer of shorter pieces" , "Writer of little prose pieces" .)