Pen inspiring love with popular invention (7)
I believe the answer is:
coinage
'invention' is the definition.
(coinage is a kind of invention)
'pen inspiring love with popular' is the wordplay.
'pen' becomes 'cage' (I've seen this in other clues).
'inspiring' means one lot of letters goes inside another.
'love' becomes 'o' (love means zero in tennis).
'with' is a charade indicator (letters next to each other).
'popular' becomes 'in' ('in' can mean trendy).
'o'+'in'='oin'
'cage' placed around 'oin' is 'COINAGE'.
(Other definitions for coinage that I've seen before include "Royal Mint's output" , "Fresh bit of language" , "Invention of a new word or phrase, say" , "Fresh term" , "Metal money" .)