Boast northern headgear (5)
I believe the answer is:
crown
'headgear' is the definition.
(I know that crown is a type of headgear)
'boast northern' is the wordplay.
'boast' becomes 'crow' (to crow about something is to boast about it).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'crow'+'n'='CROWN'
(Other definitions for crown that I've seen before include "Royal headdress" , "sporting award" , "Symbol of royalty" , "'Uneasy lies the head that wears a . . . . .'" , "Diadem" .)