Trial — Hampshire river (4)
I believe the answer is:
test
I believe this is a double definition.
'trial' is the first definition.
(I know that trial can be written as test)
'hampshire river' is the second definition.
(River Test)
(Other definitions for test that I've seen before include "Experiment" , "Trial; exam" , "big game" , "Critical examination or trial of qualities or abilities" , "Audition" .)