Question a witch, say (5)
I believe the answer is:
which
'question' is the definition.
'a witch say' is the wordplay.
'say' indicates a 'sounds like' (homophone) clue.
'witch' is a homophone of 'WHICH'.
(Other definitions for which that I've seen before include "What one?" , "Organisation giving information to consumers" , "Consumer magazine" .)