Reacting to oxygen in sport with sharp pain (7)

I believe the answer is:

rusting

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'reacting to oxygen' is the definition.
I can't tell whether this definition defines the answer.

'sport with sharp pain' is the wordplay.
'sport' becomes 'RU' (rugby union).
'with' says to put letters next to each other.
'sharp pain' becomes 'sting' (I've seen this before).
'ru'+'sting'='RUSTING'

'in' acts as a link.

Can you help me to learn more?

(Another definition for rusting that I've seen is " Corroding".)

I've seen this clue in The Times.

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