Lacking nucleus, fermenting yeast is swelling (4)

I believe the answer is:

stye

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'is swelling' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

(Other definitions for stye that I've seen before include "Eyelid sore" , "irritation to the viewer" , "It pains viewer" , "Inflamed swelling on the edge of an eyelid" , "Eye sore" .)

I've seen this clue in The Independent.

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