Squat in Essex river (6)
I believe the answer is:
crouch
This clue is a double definition.
'squat' is the first definition.
(to crouch is to squat down)
'essex river' is the second definition.
(the River Crouch in Essex)
'in' is the link.
(Other definitions for crouch that I've seen before include "Get down low" , "Squat" , "Hunker down" , "Bend low, preparing to spring" , "Duck" .)