At fifty-one pence, a mere impertinence (3)
I believe the answer is:
lip
'a mere impertinence' is the definition.
'lip' can be an answer for 'impertinence' (lip is a kind of impertinence). I'm unsure of the 'a mere' bit.
'at fifty-one pence' is the wordplay.
'at' is a charade indicator (letters next to each other).
'fifty one' becomes 'LI' (Roman numerals).
'pence' becomes 'p' ().
'li'+'p' is 'LIP'.
Can you help me to learn more?
(Other definitions for lip that I've seen before include "Impudent talk from part of the face" , "Rim; impertinence" , "Insolence (slang)" , "Oral impudence?" , "Aural impudence" .)