Champion losing weight, only what trainer requires (9)

I believe the answer is:

innersole

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'trainer requires' is the definition.
I can't tell whether this definition defines the answer.

'champion losing weight only' is the wordplay.
'champion' becomes 'winner' (winner of a competition).
'losing' suggests deleting specific letters.
'weight' becomes 'w' (short for weight).
'only' becomes 'sole' (synonyms).
'winner' with 'w' taken away is 'inner'.
'inner'+'sole'='INNERSOLE'

'what' is the link.

Can you help me to learn more?

I've seen this clue in The Independent.

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