One proverbial source of corporate strength (5)

I believe the answer is:

unity

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'one' is the definition.
(I know that one can be written as unity)

I don't understand the rest of the clue.

(Other definitions for unity that I've seen before include "Accord" , "Solidarity" , "Number one" , "State of togetherness" , "Mitford girl" .)

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

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