The custom of riding? (5)

I believe the answer is:

habit

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'the custom' is the definition.
(habit is a kind of custom)

I don't understand the rest of the clue.

(Other definitions for habit that I've seen before include "Usual practice; garment" , "Custom, nun's wear" , "Monk's robe or repetitive action" , "Custom - costume" , "Breaking this is hard" .)

I've seen this clue in the Evening Standard.

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