Person who agrees with fool put down (8)
I believe the answer is:
assenter
'person who agrees' is the definition.
(someone who assents to something)
'with fool put down' is the wordplay.
'with' says to put letters next to each other.
'fool' becomes 'ass' (both can mean a stupid person).
'put down' becomes 'enter' (synonyms).
'ass'+'enter' is 'ASSENTER'.
(Other definitions for assenter that I've seen before include "he's quite agreeable" , "One who agrees" .)