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' shows that the letters should be reversed in order.
'letter' becomes 'missive' (synonyms).
'rep' back-to-front 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" .)