Revived zoo topic about rule regarding simple animals (9)

'revived' is the definition.
Although both the answer and definition are adjectives, I don't see how they can define each other.

'zoo topic about rule regarding simple animals' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'zoo' is 'ozo' which is within the answer.
'about' could be 'c' (abbreviation for 'circa') and 'c' is present in the answer.
'rule' could be 'r' (abbreviation for rule) and 'r' is found within the answer.
This explanation may well be incorrect...

Can you help me to learn more?