Great sporting occasion involving cycling (4)

I believe the answer is:

mega

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'great' is the definition.
(old-fashioned UK slang for 'very good')

I don't understand the remainder of the clue.

(Other definitions for mega that I've seen before include "Fantastic!" , "Game (anag) -- great" , "Huge (slang)" , "wicked" , "Smashing" .)

I've seen this clue in The Guardian.

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