Reckoning mountaineer will flail about (11)

I believe the answer is:

enumeration

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

'reckoning' is the definition.
(I know that reckoning can be written as enumeration)

'mountaineer will flail about' is the wordplay.
'mountaineer' anagrams to 'ENUMERATION'.
Though, I am not sure how the anagram is indicated.

Can you help me to learn more?

(Other definitions for enumeration that I've seen before include "The act of numbering" , "Counting process" , "The act of mentioning one by one" , "Act of counting" .)

I've seen this clue in The Everyman puzzle in The Observer.

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