Fool not involved with weight (9)
I believe the answer is:
simpleton
'fool' is the definition.
(I've seen this before)
'not involved with weight' is the wordplay.
'not involved' becomes 'simple' (I can't explain this - if you can you should give a lot more credence to this answer).
'with' means one lot of letters go next to another.
'weight' becomes 'ton' (unit of weight).
'simple'+'ton'='SIMPLETON'
Can you help me to learn more?
(Other definitions for simpleton that I've seen before include "Gullible type" , "Dunce" , "Idiot" , "Ass" , "Nincompoop" .)