One in 1,000? (5)
I believe the answer is:
comma
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">one in 1000?</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)