Newcastle's river (4)
I believe the answer is:
tyne
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">newcastle's river</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for tyne that I've seen before include "River of NE England" , "River of Northumberland" , "Banker" , "River running through Newcastle" , "North of England river" .)