Run to avoid 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.
(type of snake)

I cannot understand how the remainder of the clue works.

(Other definitions for adder that I've seen before include "1 of 21 [SNAKES]" , "An arithmetic snake" , "calculator?" , "Dread (anag)" , "Something you have to reckon with" .)

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

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.

Connecting...