Crossword Genius

At fifty-one pence, a mere impertinence (3)

Ross

I believe the answer is:

lip

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in the Evening Standard.
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