River at Runnymede (6)
I believe the answer is:
thames
'river at runnymede' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for thames that I've seen before include "River at Goring" , "River at Windsor" , "The Ma's in London" , "Longest river in England" , "Windsor's river" .)