A weapon was tested again after backfiring (5)
I believe the answer is:
taser
'a weapon' is the definition.
(taser is a type of weapon)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for taser that I've seen before include "The weapon delivers a temporary paralysing electric shock" , "Electric police weapon" , "Weapon firing electrified darts; stare (anag.)" , "Device using electrified darts to stun" , "A shocking thing" .)