Divide northern river (6)
I believe the answer is:
severn
'river' is the definition.
(I've seen this before)
'divide northern' is the wordplay.
'divide' becomes 'sever' (severing is a kind of dividing).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'sever'+'n'='SEVERN'
(Other definitions for severn that I've seen before include "Major UK river" , "Shrewsbury's river" , "Longest British river" , "River rising in Wales" , "River with famous bore" .)