Easy to put on weight for fool (9)
I believe the answer is:
simpleton
'for fool' is the definition.
(I've seen this before)
'easy to put on weight' is the wordplay.
'easy' becomes 'simple' (similar in meaning).
'to put on' means one lot of letters go next to another.
'weight' becomes 'ton' (unit of weight).
'simple'+'ton'='SIMPLETON'
(Other definitions for simpleton that I've seen before include "Foolish or gullible person" , "Ninny" , "Simon for one" , "Nincompoop" , "Unintelligent person" .)