Reader of lesson always carries gun (9)
I believe the answer is:
epistoler
'reader of lesson' is the definition.
The definition suggests a singular noun which matches the answer.
'always carries gun' is the wordplay.
'always' becomes 'eer' (poetic way of saying ever).
'carries' is an insertion indicator (inserted letters carried inside).
'gun' becomes 'pistol' (pistol is a kind of gun).
'eer' placed around 'pistol' is 'EPISTOLER'.
Can you help me to learn more?
(Another definition for epistoler that I've seen is " Perhaps man of letters".)