Cut off, northern river (6)
I believe the answer is:
severn
'river' is the definition.
(I've seen this before)
'cut off northern' is the wordplay.
'cut off' becomes 'sever' (I've seen this before).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'sever'+'n'='SEVERN'
(Other definitions for severn that I've seen before include "British banker" , "River at Worcester" , "Major UK river" , "West Country river" , "Britain's longest river" .)