Lean film, name unknown (6)
I believe the answer is:
skinny
'lean' is the definition.
('skinny' can be similar in meaning to 'lean')
'film name unknown' is the wordplay.
'film' becomes 'skin' (I've seen this in other clues**).
'name' becomes 'n' (common abbreviation e.g. on forms).
'unknown' becomes 'y' (algebra).
'skin'+'n'+'y'='SKINNY'
(Other definitions for skinny that I've seen before include "Scrawny, underweight" , "Tight-fitting" , "Lacking food" , "Emaciated" , "Unattractively thin" .)