Expert on defending old fool (7)
I believe the answer is:
buffoon
'fool' is the definition.
(buffoon is a kind of fool)
'expert on defending old' is the wordplay.
'expert' becomes 'buff' (buff can mean an expert or aficionado).
'defending' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'on' going around 'o' is 'oon'.
'buff'+'oon'='BUFFOON'
(Other definitions for buffoon that I've seen before include "Ludicrous figure" , "Jester" , "Ridiculous person" , "comic" , "'Clown, fool (7)'" .)