Denounce professional writer (9)
I believe the answer is:
proscribe
'denounce' is the definition.
(I've seen this before)
'professional writer' is the wordplay.
'professional' becomes 'pro' ('pro' can be a synonym of 'professional').
'writer' becomes 'scribe' (scribe is a kind of writer**).
'pro'+'scribe'='PROSCRIBE'
(Other definitions for proscribe that I've seen before include "Denounce" , "Exile" , "Forbid; condemn" , "Ban" , "Prohibit" .)