Poirot's cases were never so like a baffling crossword? (8)
I believe the answer is:
unsolved
This clue is a double definition.
'poirot's cases were never so' is the first definition.
(Hercule Poirot is a fictional detective)
'like a baffling crossword?' is the second definition.
(a baffling crossword might be left unsolved)
(Other definitions for unsolved that I've seen before include "Not worked out" , "Having no answer" , "Still a puzzle like old Venus" , "Not yet explained" .)