Machine part that's good for business (3)
I believe the answer is:
cog
'machine part' is the definition.
(I know that cog is a type of machine part)
'good for business' is the wordplay.
'good' becomes 'g' (abbreviation).
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'business' becomes 'co' (short for company).
'g' after 'co' is 'COG'.
'that's' is the link.
(Other definitions for cog that I've seen before include "Machine component" , "one engaged in revolutionary activity" , "Tooth on gearwheel" , "Tooth on the edge of wheel" , "Gear-tooth" .)