A hundred up for a prize? (3)
I believe the answer is:
cup
'for a prize?' is the definition.
(cup is a kind of prize)
'a hundred up' is the wordplay.
'a hundred' becomes 'c' (Roman numeral).
'c'+'up'='CUP'
(Other definitions for cup that I've seen before include "Trophy, drink" , "Chalice; trophy" , "Beaker" , "Sort of cake" , "Container to drink from - prize" .)