Put some lemon squash in the wine and it could be fatal! (7)

I believe the answer is:

hemlock

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'put' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

I can't explain the rest of the clue.

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(Other definitions for hemlock that I've seen before include "preparation to kill" , "Highly poisonous relative of parsley" , "Socratic poison" , "Poisonous drug" , "Poisonous plant" .)

I've seen this clue in the Evening Standard.

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