Horrible experience in which mounted champion gets beheaded? (9)

I believe the answer is:

nightmare

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

'horrible experience' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for nightmare that I've seen before include "He joins migrant in scary illusion" , "Frightening dream" , "Terrifying dream" , "dreadful experience" , "Horror" .)

I've seen this clue in The Times.

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