Hopping joint? (5)
I believe the answer is:
ankle
'hopping joint?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ankle that I've seen before include "Major foot joint" , "Low joint" , "Major joint in lower limb" , "Location of the talus" , "Part of the leg between the foot and the shin" .)