Joint that with added grease helps hard work (5)
I believe the answer is:
elbow
This clue is a double definition.
'joint' is the first definition.
(joint of the body)
'that with added grease helps hard work' is the second definition.
(as in elbow grease)
(Other definitions for elbow that I've seen before include "Reject" , "Sort of grease" , "Arm joint, more power to it" , "sack?" , "Hinge joint" .)