Expert has nothing on clown (7)
I believe the answer is:
buffoon
'clown' is the definition.
('buffoon' can be a synonym of 'clown')
'expert has nothing on' is the wordplay.
'expert' becomes 'buff' (I've seen this before).
'has nothing' becomes 'o' (I've seen this in other clues).
'buff'+'o'+'on'='BUFFOON'
(Other definitions for buffoon that I've seen before include "Ridiculous person" , "Ludicrous figure" , "'Clown, fool (7)'" , "Foolish person" , "comic" .)