Pen sort of language that's found on UK banknotes (5,4)
I believe the answer is:
pound sign
'uk banknotes' is the definition.
I know nothing about this answer so I can't judge whether this works.
'pen sort of language that's found on' is the wordplay.
'pen' becomes 'pound' (both can mean an animal enclosure).
'sort of language' becomes 'sign' (sign language).
'that's found on' is a charade indicator (letters next to each other).
'pound'+'sign'='POUND SIGN'
Can you help me to learn more?
(Other definitions for pound sign that I've seen before include "PS" , "Barred character" , "Symbol for sterling" .)