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?" .)

I've seen this clue in The Times.

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