McEnroe, say, magnificent on grass (9)

I believe the answer is:

superbrat

'mcenroe say' is the definition.
(tennis player John McEnroe behaved brattishly)

'magnificent on grass' is the wordplay.
'magnificent' becomes 'superb' (I've seen this before).
'on' says to put letters next to each other.
'grass' becomes 'rat' (both can mean a snitch or informer).
'superb'+'rat'='SUPERBRAT'

(Another definition for superbrat that I've seen is " Infuriatingly, he served".)

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