Fifty-one pence? What a sauce! (3)
I believe the answer is:
lip
'what a sauce' is the definition.
(I've seen this in other clues)
'fifty-one pence?' is the wordplay.
'fifty' becomes 'l' (roman numeral).
'one' becomes 'i' (roman numeral).
'pence?' becomes 'p'.
'l'+'i'+'p'='LIP'
(Other definitions for lip that I've seen before include "giving such insincere service" , "Edge; impudence" , "Insolence (slang)" , "Aural impudence" , "Sauce" .)