River of eastern England (3)
I believe the answer is:
cam
'river of eastern england' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cam that I've seen before include "Irregular projection" , "It controls the motion of engine cylinder valves" , "running into Great Ouse?" , "Bump on a shaft for pushing another shaft" , "River of eastern England" .)