Half and half, I’m well and not well (3)

I believe the answer is:

ill

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'not well' is the definition.
('ill' is the opposite to 'well')

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

Can you help me to learn more?

(Other definitions for ill that I've seen before include "Unwell from bacillus" , "Indisposed" , "laid up?" , "state, briefly" , "Imperfectly; unwell" .)

I've seen this clue in The Times.

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