Proverbially filthy bit of luck again! (5)
I believe the answer is:
lucre
'proverbially filthy bit of luck' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'again' is the wordplay.
I cannot really see how this works, but
'again' could be 're' (prefix meaning 'again') and 're' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for lucre that I've seen before include "(Filthy) profit" , "Possession of the rich" , "Sordid gain; riches" , "profits" , "ill-gotten gain" .)