Easy to put on weight for a fool (9)
I believe the answer is:
simpleton
'for a fool' is the definition.
(I've seen this in another clue)
'easy to put on weight' is the wordplay.
'easy' becomes 'simple' (associated 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 "Simon for one" , "Naive person" , "Halfwit" , "Unintelligent person" , "Idiot" .)