He is placed second in this (8,5,2,3,8)

I believe the answer is:

periodic table of the elements

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'this' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'he is placed second' is the wordplay.
I cannot really see how this works, but
'he' could be 'tab' and 'tab' is present in the answer.
'second' could be 's' (common abbreviation) and 's' is found within the answer.
This explanation may well be incorrect...

'in' is the link.

Can you help me to learn more?

I've seen this clue in The Guardian.

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