The puzzle that used to strain (6)
I believe the answer is:
riddle
This clue could be a double definition.
'the puzzle that' is the first definition.
'riddle' can be an answer for 'the puzzle' (a riddle is a puzzle or problem). I'm unsure of the remainder of the definition.
'to strain' is the second definition.
(I know that riddle is a more specific form of the action sieve)
'used' acts as a link.
Can you help me to learn more?
(Other definitions for riddle that I've seen before include "mystery" , "Word puzzle or coarse sieve" , "it's full of holes" , "Cryptic statement" , "Sieve or conundrum" .)