Lenient – or by the letter! (10)
I believe the answer is:
permissive
'lenient' is the definition.
(similar in meaning)
'by the letter' is the wordplay.
'by' becomes 'per' (I've seen this in another clue).
'the letter' becomes 'missive' ('missive' can be a synonym of 'letter').
'per'+'missive'='PERMISSIVE'
'or' acts as a link.
(Other definitions for permissive that I've seen before include "Lenient, indulgent" , "Liberal, broad-minded" , "Showing indulgence" , "granting authorisation" , "Free" .)