Old sewer's part altered having collapsed (7)
I believe the answer is:
treadle
'old sewer's part' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'altered having collapsed' is the wordplay.
'altered' can be anagrammed to 'TREADLE'.
Though, I'm not sure how this is indicated.
Can you help me to learn more?
(Other definitions for treadle that I've seen before include "Pedal operating machine" , "Foot pedal for power" , "Rocking lever operated by foot" , "Foot lever that works a machine" , "Bit of machine" .)