Measure some adopt, I challenge (5)

I believe the answer is:

optic

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

'some adopt i challenge' is the wordplay.
'some' indicates a hidden word (some parts of the word are used).
'OPTIC' is hidden within 'adopt i challenge'.

Can you help me to learn more?

(Other definitions for optic that I've seen before include "Relating to sight" , "Pub drinks dispenser" , "Of sight or the eye" , "Bar measure" , "Eye - spirit measure" .)

I've seen this clue in The Times.

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