The defined seriousness of density (8,7)
I believe the answer is:
specific gravity
'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?".)