Being already bad, is unable to go off! (3)
I believe the answer is:
dud
'being' is the definition.
(dud is a kind of being)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for dud that I've seen before include "Palindrome for a flop" , "Ineffectual thing/person" , "Explosive not detonating" , "Broken" , "Bullet that fails to fire" .)