Prohibited argument for writer beginning to defame (10)
I believe the answer is:
proscribed
'prohibited argument' is the definition.
'proscribed' can be an answer for 'prohibited' ('proscribe' can be a synonym of 'prohibit'). I am unsure of the 'argument' bit.
'for writer beginning to defame' is the wordplay.
'for' becomes 'pro' (as in being 'pro' something).
'writer' becomes 'scribe' (scribe is a kind of writer).
'beginning to' indicates taking the first letters.
The first letter of 'defame' is 'd'.
'pro'+'scribe'+'d'='PROSCRIBED'
Can you help me to learn more?
(Other definitions for proscribed that I've seen before include "outlawed" , "on the banned list" , "Forbidden" .)