Crossword Genius

By which the fortunate swimmer won the competition? (6,2,4)

Ross

I believe the answer is:

stroke of luck

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

'fortunate swimmer won the competition?' is the definition.
I know nothing about this answer so I can't tell whether this works.

'by which the' is the wordplay.
I cannot quite see how this works, but
'by' could be 'of' and 'of' is found within the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Another definition for stroke of luck that I've seen is " It is fortuitous and fortunate".)

I've seen this clue in the Evening Standard.
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