Bad, bad character losing head (4)

I believe the answer is:

evil

I'm a little stuck... Click here to teach me more about this clue!

'bad' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

(Other definitions for evil that I've seen before include "Extremely bad" , "Very wicked or bad" , "Extremely unpleasant" , "Wicked - social problem" , "Live badly in depravity" .)

I've seen this clue in The Telegraph.

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