After lengthy period, I leave French city displaying corruption (9)
I believe the answer is:
decadence
'corruption' is the definition.
(I've seen this before)
'after lengthy period i leave french city' is the wordplay.
'after' says to put letters next to each other.
'lengthy period' becomes 'decade' (decade is a kind of period. I am not sure about the 'lengthy' bit.).
'leave' indicates named letters should be taken away.
'french city' becomes 'nice' (Nice is a French city).
'nice' with 'i' taken out is 'nce'.
'decade' put next to 'nce' is 'DECADENCE'.
'displaying' is the link.
Can you help me to learn more?
(Other definitions for decadence that I've seen before include "Degeneration" , "Corruption" , "Moral or cultural decline" , "Immoral behaviour" , "Decline in quality" .)