Easy to put on weight for an idiot (9)
I believe the answer is:
simpleton
'an idiot' 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'
'for' is the link.
(Other definitions for simpleton that I've seen before include "One very slow" , "Nincompoop" , "Unintelligent person" , "Natural" , "Blockhead" .)