River cut north (6)
I believe the answer is:
severn
'river' is the definition.
(I've seen this before)
'cut north' is the wordplay.
'cut' becomes 'sever' (severing is a kind of cutting).
'north' becomes 'n' ('N' can be a synonym of 'north').
'sever'+'n'='SEVERN'
(Other definitions for severn that I've seen before include "Longest British river" , "River with famous bore" , "River flowing into the Bristol Channel" , "Shrewsbury river" , "GB's longest river" .)