Roman conspirator screening a mostly banal film (10)
I believe the answer is:
casablanca
'film' is the definition.
(I have seen 'romantic movie' mean 'casablanca' so perhaps 'movie' could also mean 'casablanca')
'roman conspirator screening a mostly banal' is the wordplay.
'roman conspirator' becomes 'casca' (I've seen this in another clue).
'screening' means one lot of letters goes inside another (inserted letters are covered or screened by others).
'mostly banal' becomes 'blan' (I can't justify this - if you can you should give a lot more credence to this answer).
'a'+'blan'='ablan'
'casca' placed around 'ablan' is 'CASABLANCA'.
Can you help me to learn more?
(Other definitions for casablanca that I've seen before include "Bogart movie" , "Moroccan port - 1942 Bogart/Bergman film" , "Moroccan city; film" , "Famous Bogart film" , "1942 film starring Bogart and Bergman" .)