Habit of Romans to blockade northern kingdom (5)
I believe the answer is:
tonga
'kingdom' is the definition.
(kingdom in Polynesia)
'habit of romans to blockade northern' is the wordplay.
'habit of romans' becomes 'toga' (roman garment).
'to blockade' is an insertion indicator.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'toga' enclosing 'n' is 'TONGA'.
(Other definitions for tonga that I've seen before include "Pacific island kingdom" , "part of Polynesia" , "group of Pacific islands" , "two-wheeler" , "Polynesian monarchy, member of the Commonwealth" .)