Headgear on back of the head (4)
I believe the answer is:
cape
'head' is the definition.
(thesaurus)
'headgear on back of the' is the wordplay.
'headgear' becomes 'cap' (cap is a kind of headgear**).
'on' says to put letters next to each other.
'back of' indicates one should take the final letters.
The last letter of 'the' is 'e'.
'cap'+'e'='CAPE'
(Other definitions for cape that I've seen before include "Batman has one" , "Cloak, robe" , "Sleeveless garment like a cloak" , "Wrath perhaps" , "Garment - promontory" .)