Joint, about a pound, lifted sadness (5)
I believe the answer is:
elbow
'joint' is the definition.
(elbow joint is a kind of joint)
'about a pound lifted sadness' is the wordplay.
'about' means one lot of letters goes inside another.
'a pound' becomes 'lb' ('lb' can be a synonym of 'pound'**).
'lifted' is a reversal indicator (going up in down clue).
'sadness' becomes 'woe' (woe is a kind of sadness).
'woe' reversed gives 'eow'.
'lb' inserted into 'eow' is 'ELBOW'.
(Other definitions for elbow that I've seen before include "Reject" , "Arm joint, more power to it" , "part of the body" , "Dismissal?" , "Major arm joint" .)