Run to avoid large snake (5)
I believe the answer is:
adder
'snake' is the definition.
(adder is a kind of snake)
I can't explain the remainder of the clue.
Can you help me to learn more?
(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" .)