Neither may come from “19 Across” (3)

I believe the answer is:

nor

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'neither' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

(Other definitions for nor that I've seen before include "Conjunction meaning 'and not'" , "And not either" , "Common conjunction, goes with neither" , "Negative conjunction" , "Neither pairing" .)

I've seen this clue in the Evening Standard.

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