Clue: Approximately 1000 top best in revolt — it's a really nasty place to be (9)
Answer: CACOTOPIA
Explanation: Definition: a really nasty place to be
A charade of CA (*circa*, 'approximately') plus C (Roman numeral, '100'-) plus O (-'0') plus 'top' plus IA, a reversal ('in revolt') of AI (A-1, 'best').