Gateshead's river (4)
I believe the answer is:
tyne
'gateshead's river' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for tyne that I've seen before include "Gateshead's river" , "Serve as evidence for" , "Banker" , "River at Gateshead" , "River of Northumberland" .)