Unknown Parisian is ready to play Beatles song (9)
I believe the answer is:
yesterday
'beatles song' is the definition.
(I've seen this before)
'unknown parisian is ready to play' is the wordplay.
'unknown' becomes 'y' (algebra).
'parisian is' becomes 'est' ('is' in French).
'to play' is an anagram indicator.
'ready' is an anagram of 'erday'.
'y'+'est'+'erday'='YESTERDAY'
(Other definitions for yesterday that I've seen before include "24-hours ago" , "Lennon and McCartney classic" , "24 hours ago - hit for The Beatles" , "of most recent history" , "Much-recorded song" .)