Pen round old popular invention (7)
I believe the answer is:
coinage
'invention' is the definition.
(coinage is a kind of invention)
'pen round old popular' is the wordplay.
'pen' becomes 'cage' (I've seen this before).
'round' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'popular' becomes 'in'.
'o'+'in'='oin'
'cage' going around 'oin' is 'COINAGE'.
(Other definitions for coinage that I've seen before include "Invention of a new word or phrase, say" , "Some ready" , "Metal currency" , "Potential for change" , "Fresh bit of language" .)