Peter out of line dismissing arithmetic? (5)

I believe the answer is:

cease

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'peter out' is the definition.
The definition and answer can be both to do with fundamental existence as well as being verbs in their base form.
Maybe they are linked in a way I don't understand?

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for cease that I've seen before include "Desist" , "Finish, end" , "Stop, terminate" , "Discontinue" , "Stop, finish" .)

I've seen this clue in The Irish Times.

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