Sister's daughter (5)
I believe the answer is:
niece
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sister's daughter</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for niece that I've seen before include "Family relation" , "Criminal" , "Brother or sister's daughter" , "Son's cousin" , "A sibling's daughter" .)