Excellent game was played (4)

I believe the answer is:

mega

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

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

'game was played' is the wordplay.
'game' anagrams to 'MEGA'.
But, I am not clear how this is indicated.

Can you help me to learn more?

(Other definitions for mega that I've seen before include "Very large (colloq.)" , "Great - denoting a million" , "Great American" , "Huge (slang)" , "Game (anag) - excellent" .)

I've seen this clue in The Mirror.

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