A screen mystery (6)
I believe the answer is:
riddle
This clue is a double definition.
'a screen' is the first definition.
(I know that riddle is a type of screen)
'mystery' is the second definition.
(I've seen this before)
(Other definitions for riddle that I've seen before include "Run through" , "Kind of puzzle or coarse sieve" , "sort out" , "it's full of holes" , "Gradual assimilation of ideas or knowledge" .)