At fifty-one pence, it's an insult! (3)
I believe the answer is:
lip
'an insult' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'at fifty-one pence' is the wordplay.
'at' means one lot of letters go next to another.
'fifty' becomes 'l' (Roman numeral).
'one' becomes 'i' (Roman numeral).
'pence' becomes 'p'.
'l'+'i' is 'li'.
'li'+'p'='LIP'
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for lip that I've seen before include "Sauce" , "Aural impudence" , "giving such insincere service" , "Brim" , "Rim; impertinence" .)