Crossword Genius

The defined seriousness of density (8,7)

Ross

I believe the answer is:

specific gravity

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'density' is the definition.
(specific gravity is a kind of density)

'the defined seriousness' is the wordplay.
'the defined' becomes 'specific' (I can't explain this - if you can you should give a lot more credence to this answer).
'seriousness' becomes 'gravity' (gravity can mean seriousness or urgency).
'specific'+'gravity'='SPECIFIC GRAVITY'

'of' is the link.

Can you help me to learn more?

(Another definition for specific gravity that I've seen is " relative to chemists?".)

I've seen this clue in the Sydney Morning Herald.
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