Infirm salesperson appears during month with it (8)
I believe the answer is:
decrepit
'infirm' is the definition.
(I know that infirm can be written as decrepit)
'salesperson appears during month with it' is the wordplay.
I cannot really understand how this works, but
'salesperson' could be 'rep' (sales rep) and 'rep' is located in the answer.
'month' could be 'dec' (abbreviation for December) and 'dec' is found in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
A single letter 'i' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for decrepit that I've seen before include "Depicter in state of dilapidation" , "In the last stages of decay" , "run down" , "Frail and creaky" , "Old and infirm" .)