Stretch of Hampshire river? (4)

I believe the answer is:

test

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'hampshire river?' is the definition.
(River Test)

I don't understand how the remainder of the clue works.

(Other definitions for test that I've seen before include "Trial; exam" , "Evaluation" , "Sample" , "Dip one's toe in" , "Examine to play international cricket match" .)

I've seen this clue in The Sun.

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