Decline in quality of French rhythm (9)
I believe the answer is:
decadence
'decline in quality' is the definition.
(I've seen this before)
'of french rhythm' is the wordplay.
'of french' becomes 'de' ('of' in French).
'rhythm' becomes 'cadence' (cadence is a kind of rhythm).
'de'+'cadence'='DECADENCE'
(Other definitions for decadence that I've seen before include "depravity" , "Corruption" , "Progressive decline" , "State of immorality and self-indulgence" , "Perishing state" .)