Slate prudish article penned by socialist (9)
I believe the answer is:
reprimand
'slate' is the definition.
(thesaurus)
'prudish article penned by socialist' is the wordplay.
'prudish' becomes 'prim' ('prim' can be a synonym of 'prudish').
'article' becomes 'an' (indefinite article).
'penned by' is an insertion indicator.
'socialist' becomes 'red' (red is a kind of socialist).
'prim'+'an'='priman'
'priman' put within 'red' is 'REPRIMAND'.
(Other definitions for reprimand that I've seen before include "Tick off - carpet" , "Severe reproof" , "Expression of disapproval" , "Admonish" , "telling off" .)