Old explosive device used for blasting holes in gates and walls (with which one should avoid hoisting oneself!) (6)
I believe the answer is:
petard
(Other definitions for petard that I've seen before include "Small bomb; firework" , "Explosive device (with which one may be hoist)" , "A small bomb - don't be hoist by your own" , "charged case" , "A small bomb - you may be hoist with your own" .)