Am I certain this dispersed adders? (14)

I believe the answer is:

arithmeticians

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'adders?' is the definition.
Although both the answer and definition are plural nouns, I can't understand how they can define each other.

'am i certain this dispersed' is the wordplay.
'dispersed' is an anagram indicator.
'am'+'i'+'certain'+'this'='amicertainthis'
'amicertainthis' is an anagram of 'ARITHMETICIANS'.

Can you help me to learn more?

(Other definitions for arithmeticians that I've seen before include "number-crunchers" , "people good at sums" .)

I've seen this clue in The Sunday Times.

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