Backed up, he's still no good (3)

I believe the answer is:

dud

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'backed up he's' is the definition.
'Dud' can be an answer for 'he's'. I'm not sure about the 'backed up' bit.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for dud that I've seen before include "Not working" , "Faulty" , "One failing to work, explode" , "Broken" , "Explosive not detonating" .)

I've seen this clue in the Evening Standard.

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