Filter problem (6)
I believe the answer is:
riddle
I believe this clue is a double definition.
'filter' is the first definition.
(riddle is a type of sieve or filter)
'problem' is the second definition.
(riddle is a kind of problem)
(Other definitions for riddle that I've seen before include "Cryptic statement" , "Cryptic question" , "Brain-teaser and coarse sieve" , "it's full of holes" , "Eg, When is a door not a door?" .)