Playing one's best against spinner for Middlesex, initially (2,3,4)

I believe the answer is:

on top form

'playing one's best' is the definition.
(form can mean someone's quality of play)

'against spinner for middlesex initially' is the wordplay.
'against' becomes 'on' (I've seen this in other clues).
'spinner' becomes 'top' (something that spins).
'initially' indicates taking the first letters.
The initial letter of 'middlesex' is 'm'.
'on'+'top'+'for'+'m'='ON TOP FORM'

I've seen this clue in The Times.

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