Herb's gained weight - the fool (9)

I believe the answer is:

simpleton

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

'herb's gained weight' is the wordplay.
'herb's gained' becomes 'simple' (I can't justify this - if you can you should believe this answer much more).
'weight' becomes 'ton' (unit of weight).
'simple'+'ton'='SIMPLETON'

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(Other definitions for simpleton that I've seen before include "Ninny" , "Simon for one" , "Dunce" , "Ass" , "Naive person" .)

I've seen this clue in The Globe and Mail.

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