Almost all mountain cycling is hard to endure (6)

I believe the answer is:

severe

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'hard to endure' is the definition.
(I've seen this in another clue)

I don't understand how the remainder of the clue works.

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(Other definitions for severe that I've seen before include "Stern or stringent" , "Extreme and difficult to endure" , "Hard to endure" , "Strict and unbending" , "Acute" .)

I've seen this clue in The Independent.

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