Old-fashioned origin of a mathematical function (6,4)

I believe the answer is:

square root

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'a mathematical function' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

'old-fashioned origin' is the wordplay.
'old fashioned' becomes 'square'.
'origin' becomes 'root' (synonyms).
'square'+'root'='SQUARE ROOT'

'of' acts as a link.

Can you help me to learn more?

(Other definitions for square root that I've seen before include "Mathematical term" , "2 of 4, perhaps" , "For one, one" .)

I've seen this clue in The Globe and Mail.

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