Political doctrine on the subject of class is principle of Marxism (9)
I believe the answer is:
reformism
'principle of marxism' is the definition.
The definition suggests a singular noun which matches the answer.
'political doctrine on the subject of class' is the wordplay.
'political doctrine' becomes 'ism' (I've seen this in another clue).
'on' means one lot of letters go next to another.
'the subject of' becomes 're' ('re' means regarding or about).
'class' becomes 'form' (synonyms).
're'+'form'='reform'
'ism' after 'reform' is 'REFORMISM'.
'is' is the link.
Can you help me to learn more?
(Another definition for reformism that I've seen is " policy of improvement".)