A question of western headgear (4)
I believe the answer is:
what
'a question of western headgear' is the definition.
I can't judge whether this defines the answer.
'western headgear' is the wordplay.
'western' becomes 'w'.
'headgear' becomes 'hat' (hat is a kind of headgear).
'w'+'hat'='WHAT'
Can you help me to learn more?
(Other definitions for what that I've seen before include "Eh?" , "I didn't hear" , "Which thing?" , "That which - come again!" , "Interrogatory word" .)