Corrupt villain beginning to ebb into depression (8)
I believe the answer is:
decadent
'corrupt' is the definition.
(I've seen this before)
'villain beginning to ebb into depression' is the wordplay.
'villain' becomes 'cad' (cad can mean a nasty person).
'beginning to' says to take the initial letters.
'into' is an insertion indicator.
'depression' becomes 'dent' (dent is a kind of depression).
The first letter of 'ebb' is 'e'.
'cad'+'e'='cade'
'cade' going inside 'dent' is 'DECADENT'.
(Other definitions for decadent that I've seen before include "base?" , "Marked by excessive self-indulgence and moral decay" , "Luxuriously self-indulgent" , "Decanted in corrupt way" , "Morally corrupt" .)