Cut off the northern river (6)
I believe the answer is:
severn
'river' is the definition.
(River Severn)
'cut off the northern' is the wordplay.
'cut off' becomes 'sever' (to sever is to cut).
'the northern' becomes 'n' (abbreviation e.g. N. Ireland).
'sever'+'n'='SEVERN'
(Other definitions for severn that I've seen before include "British banker" , "River of England and Wales" , "Welsh and English river" , "River at Gloucester" , "River rising in Wales" .)