Wide headgear, eh? (4)
I believe the answer is:
what
'eh?' is the definition.
('eh' can mean 'pardon' or 'what')
'wide headgear' is the wordplay.
'wide' becomes 'w' (abbreviation e.g. in cricket).
'headgear' becomes 'hat' (hats are a type of headgear).
'w'+'hat'='WHAT'
(Other definitions for what that I've seen before include "Which thing?" , "Eh?" , "Word used in a question" , "I didn't hear" , "That which - come again!" .)