Except when included in gun lessons (6)
I believe the answer is:
unless
'except when' is the definition.
('except when' can mean 'unless')
'included in gun lessons' is the wordplay.
'included in' indicates a hidden word.
'UNLESS' is hidden in the letters of 'gun lessons'.
(Other definitions for unless that I've seen before include "Except on the condition that" , "bar" , "Otherwise than if" , "Conjunction meaning except when" , "Except, save" .)