Pound invested in misery served up in joint (5)
I believe the answer is:
elbow
'joint' is the definition.
(elbow joint is a kind of joint)
'pound invested in misery served up' is the wordplay.
'pound' becomes 'lb' ('lb' can be a synonym of 'pound').
'invested in' means one lot of letters goes inside another.
'misery' becomes 'woe' (woe is a kind of misery).
'served up' is a reversal indicator.
'woe' backwards is 'eow'.
'lb' going inside 'eow' is 'ELBOW'.
'in' acts as a link.
(Other definitions for elbow that I've seen before include "Arm joint, more power to it" , "sack?" , "Dismissal?" , "sharp turn" , "Major arm joint" .)