Sift something difficult to understand (6)
I believe the answer is:
riddle
This clue could be a double definition.
'sift' is the first definition.
(I know that riddle is a more specific form of sift)
'understand' is the second definition.
(I know that riddle is a more specific form of figure out)
'something difficult to' is the link.
This may not be right. Some or all of it may belong to one of the definitions.
Can you help me to learn more?
(Other definitions for riddle that I've seen before include "Brain-teaser and coarse sieve" , "Ingenious puzzle - sieve" , "Make many openings" , "sort out" , "Cryptic statement" .)