Crossword Genius

By luck, perchance? (3)

Ross

I believe the answer is:

per

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'by' is the definition.
(I've seen this in another clue)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for per that I've seen before include "Preposition indicating by means of" , "According to" , "For each or every" , "Preposition meaning for each" .)

I've seen this clue in the Evening Standard.
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