One working on material through which Johnson led his MPs, according to Starmer (5)
I believe the answer is:
sewer
This clue could be a double definition.
'one working' is the first definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'material through which johnson led his mps according to starmer' is the second definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'on' is the link.
Can you help me to learn more?
(Other definitions for sewer that I've seen before include "Filthy channel" , "Drainage channel" , "Main drain" , "Tailor" , "Pipe for carrying away waste matter" .)