Challenge Hampshire runner (4)
I believe the answer is:
test
I believe this clue is a double definition.
'challenge' is the first definition.
(I've seen this in another clue)
'hampshire runner' is the second definition.
(River Test in Hampshire)
(Other definitions for test that I've seen before include "Trial; exam" , "Dip one's toe in" , "big game" , "Assay" , "Critical examination or trial of qualities or abilities" .)