Separate name for river (6)
I believe the answer is:
severn
'river' is the definition.
(I've seen this before)
'separate name' is the wordplay.
'separate' becomes 'sever' (severing is a kind of separating).
'name' becomes 'n' (common abbreviation e.g. on forms).
'sever'+'n'='SEVERN'
'for' is the link.
(Other definitions for severn that I've seen before include "Shrewsbury river" , "Shrewsbury's river" , "Running water" , "River of England and Wales" , "River at Worcester" .)