Upset poppy sellers, pinching one -- that's the road to ruin (8,5)

I believe the answer is:

slippery slope

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But I can't explain it at all! Could you help me understand?

(Other definitions for slippery slope that I've seen before include "Downward-leading course" , "dangerous course" , "Restoration to life" , "Course leading to a bad end" , "Course likely to lead one into trouble" .)

I've seen this clue in The Telegraph.

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