Approximately 1000 top best in revolt — it's a really nasty place to be (9)

'a really nasty place to be' is the definition.
(term for the worst place imaginable)

'approximately 1000 top best in revolt' is the wordplay.
I cannot really understand how this works, but
'approximately' could be 'ca' (abbreviation for circa) and 'ca' is found in the answer.
'best' could be 'top' (top can mean the very best) and 'top' is found in the answer.
This may be the basis of clue (or it may be nonsense).

'it's' is the link.