Perhaps the lightest jockey is bet on (7)

I believe the answer is:

boniest

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'perhaps the lightest' is the definition.
I can't tell whether this defines the answer.

'jockey is bet on' is the wordplay.
'jockey' indicates an anagram (the letters scramble or jockey around).
'is'+'bet'+'on'='isbeton'
'isbeton' with letters rearranged gives 'BONIEST'.

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(Another definition for boniest that I've seen is " Most lean".)

I've seen this clue in The Independent.

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