Liverpool's river (6)
I believe the answer is:
mersey
'liverpool's river' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for mersey that I've seen before include "River with ferry and tunnel" , "River flowing into Liverpool Bay" , "Route between Liverpool and the Wirral" , "Liverpool's river" , "River rising in Derbyshire and flowing into the Irish Sea" .)