Indulgent salesman returns letter (10)
I believe the answer is:
permissive
'indulgent' is the definition.
(similar in meaning)
'salesman returns letter' is the wordplay.
'salesman' becomes 'rep' (sales representative).
'returns' is a reversal indicator.
'letter' becomes 'missive' (synonyms).
'rep' in reverse letter order is 'per'.
'per'+'missive'='PERMISSIVE'
(Other definitions for permissive that I've seen before include "Liberal, broad-minded" , "Free" , "Like our society" , "Showing indulgence" , "Lenient, indulgent" .)