Snakes in pass getting riled (4)

I believe the answer is:

asps

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

'snakes' is the definition.
(asp is a kind of snake)

'pass getting riled' is the wordplay.
'pass' anagrams to 'ASPS'.
But, I am not sure how this is indicated.

'in' is the link.

Can you help me to learn more?

(Other definitions for asps that I've seen before include "Poisonous creatures" , "people" , "poisonous things" , "Deadly snakes" , "Egyptian cobras" .)

I've seen this clue in The Sun.

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