Frightful ordeal to be arming the rabble (9)

I believe the answer is:

nightmare

I'm a little stuck... Click here to teach me more about this clue!

'frightful ordeal' is the definition.
'nightmare' can be an answer for 'ordeal' (thesaurus). I am not sure about the 'frightful' bit.

'be arming the rabble' is the wordplay.
'arming the' can be anagrammed to 'NIGHTMARE'.
But, I am unsure how this is indicated.

'to' acts as a link.

Can you help me to learn more?

(Other definitions for nightmare that I've seen before include "Horrible experience" , "Terrifying experience" , "Frightening dream" , "Awful experience" , "hellish event" .)

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

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