Part of old sewer's altered after collapsing (7)
I believe the answer is:
treadle
'part of old sewer's' is the definition.
(part of an old sewing machine)
'altered after collapsing' is the wordplay.
'after collapsing' indicates an anagram (the original order collapses).
'altered' is an anagram of 'TREADLE'.
(Other definitions for treadle that I've seen before include "Rocker in a factory" , "requires footwork" , "Bit of machine" , "Foot lever working machine" , "Machine foot-lever" .)