Split up northern river (6)
I believe the answer is:
severn
'river' is the definition.
(River Severn)
'split up northern' is the wordplay.
'split up' becomes 'sever' (I've seen this in another clue).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'sever'+'n'='SEVERN'
(Other definitions for severn that I've seen before include "UK's longest river" , "Worcester's river" , "A major British river" , "River at Gloucester" , "West Country river" .)