Sinner's foremost sin (5)

I believe the answer is:

stray

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

I cannot understand how the remainder of the clue works.

(Other definitions for stray that I've seen before include "Waif and - -" , "Lost pet" , "out of place" , "Go off course" , "Wander; be unfaithful" .)

I've seen this clue in the Sydney Morning Herald.

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