Hundred pounds possibly won by an entertainer (5)
I believe the answer is:
clown
'an entertainer' is the definition.
(I know that clown is a type of jester)
'hundred pounds possibly won' is the wordplay.
'hundred' becomes 'c' (Roman numeral).
'pounds' becomes 'l' (currency sign).
'possibly' indicates an anagram.
'won' anagrammed gives 'own'.
'c'+'l'+'own'='CLOWN'
'by' acts as a link.
(Other definitions for clown that I've seen before include "stupid person" , "Chap fooling around" , "Jester" , "Foolish chap" , "Funny person in circus" .)