Puzzle strain (6)
I believe the answer is:
riddle
This clue is a double definition.
'puzzle' is the first definition.
(riddling is a kind of puzzling)
'strain' is the second definition.
(riddling is a kind of straining)
(Other definitions for riddle that I've seen before include "Kind of puzzle or coarse sieve" , "Run through" , "Sieve; enigma" , "Sphinx's conundrum?" , "Sieve or conundrum" .)