What a defender is past earning? (4,3)

I believe the answer is:

back pay

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'earning?' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'what a defender is past' is the wordplay.
'what a defender' becomes 'back' (left-backs and right-backs are football defenders).
'is past' becomes 'pay' (I can't explain this - if you can you should believe this answer much more).
'back'+'pay'='BACK PAY'

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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