Awful vision of mounted crusader getting beheaded (9)

I believe the answer is:

nightmare

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'awful vision' is the definition.
'nightmare' can be an answer for 'vision' (thesaurus). I am not certain of the 'awful' bit.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for nightmare that I've seen before include "dreadful experience" , "Awful experience" , "terrible experience" , "Frightening nocturnal fantasy" , "that might upset the rest" .)

I've seen this clue in The Independent.

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