What’s usually reduced to case of ignorance (2,3)

I believe the answer is:

id est

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'what's' is the definition.
Both the definition and answer are adverbs.
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(Other definitions for id est that I've seen before include "Namely" , "that is in Rome" , "ie (L)" , "I mean to say!" , "In other words" .)

I've seen this clue in The Times.

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