The last one's unusable for drinking (5)

I believe the answer is:

straw

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

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for straw that I've seen before include "Residue of cereal crop" , "Stalk - tube for drinking" , "Bedding material for horses etc" , "corny stuff" , "Dried corn stalks" .)

I've seen this clue in the Evening Standard.

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