What Roman wore round northern kingdom (5)
I believe the answer is:
tonga
'kingdom' is the definition.
(kingdom in Polynesia)
'what roman wore round northern' is the wordplay.
'what roman wore' becomes 'toga' (garment of ancient Rome).
'round' is an insertion indicator.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'toga' going around 'n' is 'TONGA'.
(Other definitions for tonga that I've seen before include "Polynesian monarchy, member of the Commonwealth" , "Do a bad tango in the Pacific archipelago" , "Tropic island" , "part of Polynesia" , "Island nation" .)