British river (3)
(Other definitions for dee that I've seen before include "Chester's river" , "Welsh river" , "Scottish river" , "River at Chester" , "Welsh/English river" .)
'british river' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for thames that I've seen before include "Windsor's river" , "Sam the river" , "'Old Father ___'" , "River at Lechlade" , "River at Windsor" .)