Cut north from the river (6)
I believe the answer is:
severn
'river' is the definition.
(River Severn)
'cut north' is the wordplay.
'cut' becomes 'sever' (to sever is to cut or split).
'north' becomes 'N' (abbreviation).
'sever'+'n'='SEVERN'
'from the' acts as a link.
(Other definitions for severn that I've seen before include "River at Gloucester" , "Longest UK river" , "Britian's longest river" , "GB's longest river" , "England's longest river" .)