A mystery for the big screen (6)
I believe the answer is:
riddle
This could be a double definition.
'a mystery' is the first definition.
(I've seen this before)
'big screen' is the second definition.
'riddle' can be an answer for 'screen' (synonyms). I'm not certain of the 'big' bit.
'for the' acts as a link.
Can you help me to learn more?
(Other definitions for riddle that I've seen before include "it's full of holes" , "Cryptic verse" , "mystery" , "Puzzling person, thing" , "Gradual assimilation of ideas or knowledge" .)