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Vegetable said to be very good, but nothing’s visible in this (3-6)

Ross

I believe the answer is:

pea-souper

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

(Other definitions for pea-souper that I've seen before include "Dense, dirty fog" , "Dense yellow fog" , "Thick, dirty, yellowish fog" , "Thick smokey fog" .)

I've seen this clue in The Hindu.
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