Political theorist reportedly spots first line isn't fixed (7-8)

I believe the answer is:

marxist-leninist

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'political theorist reportedly spots' is the definition.
I can't judge whether this definition defines the answer.

'first line isn't fixed' is the wordplay.
'first' becomes 'marxist' (I can't explain this - if you can you should give a lot more credence to this answer).
'fixed' is an anagram indicator.
'line'+'isnt'='lineisnt'
'lineisnt' is an anagram of 'leninist'.
'marxist'+'leninist'='MARXIST-LENINIST'

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I've seen this clue in The Telegraph.

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