Subject's temperature dropped about eye (5)

I believe the answer is:

optic

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'about eye' is the definition.
(optic means relating to eyes or vision)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for optic that I've seen before include "Relating to sight" , "connected to the eye" , "Device for dispensing measures of spirits" , "Pertaining to sight" , "Body part" .)

I've seen this clue in The Telegraph.

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