River to the Severn (4)
I believe the answer is:
avon
'river to the severn' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for avon that I've seen before include "River and former county" , "English river, flows through Stratford" , "Bristol's river" , "River of the Bard" , "River running through Bath" .)