Lethal drink knocked back roused drunk (9)

I believe the answer is:

murderous

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

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

(Other definitions for murderous that I've seen before include "Rouse drum (anag.)" , "Homicidal" , "Extremely arduous or unpleasant" , "Capable of killing" , "violent" .)

I've seen this clue in The Guardian and The Sun.

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