"Dog, second-rate individual" (that's a bit below the belt) (8)

I believe the answer is:

tailbone

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'a bit below the belt' is the definition.
I know nothing about this answer so I can't judge whether this works.

'dog second-rate individual' is the wordplay.
'dog' becomes 'tail' (synonyms).
'second rate' becomes 'b'.
'individual' becomes 'one' (an individual is one person).
'tail'+'b'+'one'='TAILBONE'

'that's' acts as a link.

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I've seen this clue in The Independent.

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