Enthusiast having nothing on is a fool (7)
I believe the answer is:
buffoon
'a fool' is the definition.
(buffoon is a kind of fool)
'enthusiast having nothing on' is the wordplay.
'enthusiast' becomes 'buff' (I've seen this before).
'having nothing' becomes 'o' (I've seen this in other clues).
'buff'+'o'+'on'='BUFFOON'
'is' is the link.
(Other definitions for buffoon that I've seen before include "Ridiculous person" , "'Clown, fool (7)'" , "comic" , "Jester" , "Ludicrous figure" .)