Great game was played (4)

I believe the answer is:

mega

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

'game was played' is the wordplay.
'game' anagrams to 'MEGA'.
However, I'm 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.)" , "'Excellent, colloquially (4)'" , "Huge (slang)" , "Fantastic!" , "Enormous; very good" .)

I've seen this clue in The Mirror.

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