Playing advantage to break John McEnroe – glad, ultimately at a loss for American (9)

'mcenroe glad ultimately at a loss for american' is the definition.
I can't judge whether this definition defines the answer.

'playing advantage to break john' is the wordplay.
'playing' becomes 'on' (on can mean playing or performing).
'advantage' becomes 'plus' (a plus is an advantage or benefit).
'to break' means one lot of letters goes inside another (inserted letters break into the word).
'john' becomes 'ned' (I can't justify this - if you can you should believe this answer much more).
'on'+'plus'='onplus'
'onplus' put within 'ned' is 'NONPLUSED'.

Can you help me to learn more?