Stockpiles containing fifty empty cartridges (6)
I believe the answer is:
blanks
'empty cartridges' is the definition.
(blanks in a gun)
'stockpiles containing fifty' is the wordplay.
'stockpiles' becomes 'banks' (bank is a kind of stockpile).
'containing' means one lot of letters goes inside another.
'fifty' becomes 'L' (Roman numeral).
'banks' going around 'l' is 'BLANKS'.
(Other definitions for blanks that I've seen before include "Vacancies" , "Explosive cartridges with no bullets" , "Cartridges with explosive charge but no bullet" .)