Wrenching of a joint (6)
I believe the answer is:
sprain
'wrenching of a joint' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sprain that I've seen before include "Muscular injury caused by wrench" , "Injure joint by wrenching it violently" , "Wrench (an ankle) 6)" , "Wrench or twist" , "Injury to the ankle, say" .)