Reflex-test target (4)
I believe the answer is:
knee
'reflex-test target' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for knee that I've seen before include "Leg bender" , "Where the femur meets the tibia" , "Leg?joint" , "This is capped by the patella" , "Hinged joint" .)