A great many retreating after appearance of very good weapon (6)
I believe the answer is:
pistol
'weapon' is the definition.
(pistol is a kind of weapon)
'a great many retreating after appearance of very good' is the wordplay.
'a great many' becomes 'lots' (many examples of something).
'retreating' says the letters should be written backwards.
'after appearance of' means one lot of letters go next to another.
'very good' becomes 'pi' (old-fashioned term for 'pious').
'lots' written backwards gives 'stol'.
'stol' put after 'pi' is 'PISTOL'.
(Other definitions for pistol that I've seen before include "Shakespearean - weapon" , "A small hand-gun" , "Rectangular" , "Small firearm" , "making an appearance in Shakespeare's King Henry V" .)