Forbidden writer given external stimulus (10)
I believe the answer is:
proscribed
'forbidden' is the definition.
('proscribe' can be a synonym of 'forbid')
'writer given external stimulus' is the wordplay.
'writer' becomes 'scribe' (scribe is a kind of writer**).
'given external' is an insertion indicator.
'stimulus' becomes 'prod' (I've seen this before).
'scribe' put into 'prod' is 'PROSCRIBED'.
(Other definitions for proscribed that I've seen before include "on the banned list" , "Forbidden" , "outlawed" .)