Soon I see one entering that’s negative (7)

I believe the answer is:

anionic

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'negative' is the definition.
Although both the answer and definition are adjectives, I don't see how they can define each other.

'soon i see one entering' is the wordplay.
'soon' becomes 'anon' (synonyms).
'see' becomes 'C' (the word for the letter, according to Chambers).
'one' becomes 'i' (Roman numeral).
'entering' is an insertion indicator.
'anon'+'i'+'c'='anonic'
'anonic' going around 'i' is 'ANIONIC'.

'that's' acts as a link.

Can you help me to learn more?

I've seen this clue in The Guardian.

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