Five hundred bet on a poet (5)
I believe the answer is:
dante
'poet' is the definition.
(13th- and 14th-century Italian poet)
'five hundred bet' is the wordplay.
'five hundred' becomes 'd' (Roman numeral).
'bet' becomes 'ante' (ante is a kind of bet).
'd'+'ante'='DANTE'
'on a' acts as a link.
(Other definitions for dante that I've seen before include "Thirteenth to fourteenth century Italian poet" , "Inferno poet" , "Net Da a poet" , "Italian poet, writer of the Divine Comedy" , "La divina commedia poet" .)