One in 1,000? (5)
I believe the answer is:
comma
'one in 1000?' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for comma that I've seen before include "Punctuation mark indicating slight pause" , "Kind of butterfly" , "Puncuation mark" , "Punctuation mark - butterfly" , "List-item separator" .)