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One working to get B+ in chemistry, say (3)

Ross

I believe the answer is:

ion

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'b in chemistry say' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

'one working' is the wordplay.
'one' becomes 'i' (Roman numeral).
'working' becomes 'on' (on can mean working or activated).
'i'+'on'='ION'

'to get' is the link.

Can you help me to learn more?

(Other definitions for ion that I've seen before include "Particle (phys.)" , "Eletrically charged particle" , "Subatomic particle" , "Electrically-charged particle" , "Electrically charged atom or group of atoms" .)

I've seen this clue in The Times.
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