Royal secret police allowed into three articles (9)
I believe the answer is:
anastasia
'royal' is the definition.
(daughter of Tsar Nicholas II of Russia)
'secret police allowed into three articles' is the wordplay.
'three articles' means three replacements for 'article'.
'secret police' becomes 'stasi' (I've seen this before).
'allowed into' means one lot of letters goes inside another.
'article' becomes 'an' (indefinite article).
'article' becomes 'a' (the indefinite article in English).
'article' becomes 'a' (the indefinite article in English).
'an'+'a'+'a'='anaa'
'stasi' put inside 'anaa' is 'ANASTASIA'.
(Other definitions for anastasia that I've seen before include "Russian princess thought once to have survived the revolution" , "Russian royal" , "Film" , "Daughter of Tsar Nicholas II once rumoured to have escaped execution" , "child of the Tsar" .)