Top bird, northern (5)
I believe the answer is:
crown
'top' is the definition.
(crown is a kind of top)
'bird northern' is the wordplay.
'bird' becomes 'crow' (crow is a kind of bird).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'crow'+'n'='CROWN'
(Other definitions for crown that I've seen before include "sporting award" , "Diadem" , "Monarch's headgear" , "Old five shilling piece" , "Coronet" .)