Most lean on one, matchless all round (7)

I believe the answer is:

boniest

'most lean' is the definition.
('bony' can be similar in meaning to 'lean')

'on one matchless all round' is the wordplay.
'one' becomes 'i' (Roman numeral).
'matchless' becomes 'best' (I've seen this in another clue).
'all round' indicates putting letters inside.
'on'+'i'='oni'
'oni' placed inside 'best' is 'BONIEST'.

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

I've seen this clue in The Times.

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