Fifty-one pence? What a sauce! (3)
I believe the answer is:
lip
'what a sauce' is the definition.
(both can mean insolence)
'fifty-one pence?' is the wordplay.
'fifty one' becomes 'LI' (Roman numerals).
'pence?' becomes 'p' ().
'li'+'p'='LIP'
(Other definitions for lip that I've seen before include "giving such insincere service" , "Edge; impudence" , "Insolence (slang)" , "Aural impudence" , "Sauce" .)