Best one puts on more weight? (6-2-4)

I believe the answer is:

second-to-none

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'best' is the definition.
(I've seen this before)

'one puts on more weight?' is the wordplay.
'puts on' means one lot of letters go next to another.
'more' becomes 'second' (I can't explain this - if you can you should believe this answer much more).
'weight?' becomes 'ton' (ton is a kind of weight).
'second'+'ton'='secondton'
'one' after 'secondton' is 'SECOND-TO-NONE'.

Can you help me to learn more?

I've seen this clue in The Telegraph.

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