It is engaged in making a machine work (3)
I believe the answer is:
cog
This clue could be a double definition.
'it is engaged in making a' is the first definition.
The definition suggests a singular noun which matches the answer.
'machine work' is the second definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe there's a link between them I don't understand?
Can you help me to learn more?
(Other definitions for cog that I've seen before include "one engaged in revolutionary activity" , "Junior official" , "Tooth on the edge of wheel" , "Tooth on the rim of gear wheel" , "Each of series of teeth on wheel" .)