Machine that's black installed in part of plant (5)
I believe the answer is:
robot
'machine' is the definition.
(I've seen this before)
'black installed in part of plant' is the wordplay.
'black' becomes 'b' (abbreviation as in pencil classifications).
'installed in' means one lot of letters goes inside another.
'part of plant' becomes 'root' (I've seen this before).
'b' going within 'root' is 'ROBOT'.
'that's' is the link.
(Other definitions for robot that I've seen before include "performer lacking emotion" , "Lifelike machine" , "Soulless worker" , "Mechanical worker" , "Programmed machine" .)