Fool with nothing on seen by expert (7)
I believe the answer is:
buffoon
'fool' is the definition.
(I know that buffoon is a type of fool)
'nothing on seen by expert' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'seen by' says to put letters next to each other.
'expert' becomes 'buff' (buff can mean an expert or aficionado).
'o'+'on'='oon'
'oon' after 'buff' is 'BUFFOON'.
'with' acts as a link.
(Other definitions for buffoon that I've seen before include "Ridiculous person" , "Foolish person" , "Ludicrous figure" , "Jester" , "comic" .)