Question commonly posed by a detective novel (8)
I believe the answer is:
whodunit
I believe this clue is a double definition.
'question commonly posed' is the first definition.
(I've seen this before)
'a detective novel' is the second definition.
(I've seen this before)
'by' is the link.
(Other definitions for whodunit that I've seen before include "American-style murder story" , "Murderer revealed at the end, US style" , "Question commonly posed" , "detective novel" , "murder investigation" .)