Firearm found after lesson is an antique (6,5)
I believe the answer is:
period piece
'an antique' is the definition.
'firearm found after lesson' is the wordplay.
'firearm' becomes 'piece' ('piece' can be a synonym of 'firearm').
'found after' means one lot of letters go next to another.
'lesson' becomes 'period'.
'piece' after 'period' is 'PERIOD PIECE'.
'is' is the link.
(Other definitions for period piece that I've seen before include "Item reminiscent of a past time" , "Item reminiscent of former times" , "old play" , "Something reminiscent of former times" , "Object belonging to a past age" .)