Crossword Genius

How do we know an asp killed Cleopatra? (5,5)

Ross

I believe the answer is:

grass snake

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

'how do' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

I don't understand the rest of the clue.

Can you help me to learn more?

(Another definition for grass snake that I've seen is " legless wild thing".)

I've seen this clue in the Evening Standard.
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