Indulgent salesman returns letter (10)
'indulgent' is the definition.
(similar in meaning)
'salesman returns letter' is the wordplay.
'salesman' becomes 'rep' (I've seen this before).
'returns' says the letters should be written in reverse.
'letter' becomes 'missive' (synonyms).
'rep' written backwards gives 'per'.
(Other definitions for permissive that I've seen before include "Liberal, broad-minded", "Free", "Like our society", "Showing indulgence", "Lenient, indulgent".)