Sibling's child (6)
(Other definitions for nephew that I've seen before include "Has a duty (to do something)" , "Brother's/sister's son" , "Family relative" , "Edward V to Richard III relation" , "Sibling's son" .)
'sibling's child' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for niece that I've seen before include "Young female relative" , "Brother's girl" , "A sibling's daughter" , "Family relation" , "girl further down the line" .)