Dose of subatomic Röntgens that's symbolised by O (7)

I believe the answer is:

omicron

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'o' is the definition.
(I've seen this in another clue)

'dose of subatomic rontgens that's symbolised' is the wordplay.
'OMICRON' is hidden within the letters.
But, I'm not sure how the hidden word is indicated.

'by' acts as a link.

Can you help me to learn more?

(Other definitions for omicron that I've seen before include "Moronic (anag) - Greek letter" , "character in Sophocles" , "Greek short 'o'" , "character from Athens" , "letter from Athens" .)

I've seen this clue in The Times.

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