Puzzle producing 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)
'producing' is the link.
(Other definitions for riddle that I've seen before include "Perforate with gunshot - poser" , "Difficult problem or coarse sieve" , "The large sieve is a puzzle" , "Gradual assimilation of ideas or knowledge" , "Eg, When is a door not a door?" .)