Worn-out salesman is seen during month with it (8)
I believe the answer is:
decrepit
'worn-out' is the definition.
(I've seen this before)
'salesman is seen during month with it' is the wordplay.
'salesman' becomes 'rep' (sales representative).
'is seen during' means one lot of letters goes inside another.
'month' becomes 'dec' (abbreviation for December).
'with' is a charade indicator (letters next to each other).
'dec'+'it'='decit'
'rep' inserted into 'decit' is 'DECREPIT'.
(Other definitions for decrepit that I've seen before include "Worn out by age or use" , "past its best" , "run down" , "Worn out; infirm" , "Dilapidated, made weak by age" .)