Speculator harangues serious investors (7)

I believe the answer is:

guesser

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'speculator' is the definition.
Both the answer and definition are singular nouns.
Perhaps there's a link between them I don't understand?

'harangues serious investors' is the wordplay.
'GUESSER' is hidden in amongst the letters.
But, I'm uncertain how this is indicated.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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