Pain in the neck - Watson's partner (5)
I believe the answer is:
crick
I believe this clue is a double definition.
'pain in the neck' is the first definition.
(I've seen this before)
'watson's partner' is the second definition.
(I've seen this in another clue)
(Other definitions for crick that I've seen before include "Twist" , "Wrench (neck or back)" , "Pain in the neck" , "Neck injury" , "Muscle spasm in the neck" .)