Item of headgear (3)
'item of headgear' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for hat that I've seen before include "Cloche or fez?" , "Headgear decoration" , "Bowler maybe" , "e.g. Pork pie" , "One to cover head" .)
(Other definitions for bowler hat that I've seen before include "weapon in a Bond film?" , "traditional topping" , "One may have felt" , "Headgear with round crown" , "Domed headgear" .)