Deadly as humans are (6)

I believe the answer is:

mortal

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'deadly' is the definition.
(synonyms)

I can't explain the rest of the clue.

(Other definitions for mortal that I've seen before include "Sound a wound is deadly" , "Subject to death, like all mankind" , "Human - transitory" , "Human being, subject to death" , "liable to die" .)

I've seen this clue in The Sun.

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