Run avoiding large snake (5)

I believe the answer is:

adder

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

'snake' is the definition.
(I know that adder is a type of viper)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for adder that I've seen before include "Totter" , "Viper common in northern Eurasia" , "Mathematical reptile" , "Dread (anag)" , "One creature" .)

I've seen this clue in The Sun.

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