Firm master’s first in class, a source of leftist propaganda once (9)

I believe the answer is:

cominform

'a source of leftist propaganda once' is the definition.
(international communist organisation 1947-1956)

'firm master's first in class' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'master's first' becomes 'm' (1st letter of 'master').
'class' becomes 'form' (synonyms).
'co'+'m'+'in'+'form'='COMINFORM'

I've seen this clue in The Times.

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