Salesman with shoestring substitute (7)
I believe the answer is:
replace
'substitute' is the definition.
('replace' can be a synonym of 'substitute')
'salesman with shoestring' is the wordplay.
'salesman' becomes 'rep' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'shoestring' becomes 'lace' (I've seen this before).
'rep'+'lace'='REPLACE'
(Other definitions for replace that I've seen before include "Provide a substitute for" , "Substitute, supersede" , "Substitute for (someone)" , "Put something back where it belongs" , "succeed" .)