Sticks in front of niece cutting sister off (7)
I believe the answer is:
inserts
'sticks in' is the definition.
('insert' can be a synonym of 'stick in')
'front of niece cutting sister off' is the wordplay.
'front of' suggests taking the first letters (the letter at the front of the word).
'cutting' is an insertion indicator (some letters cut their way into a word).
'off' is an anagram indicator.
The initial letter of 'niece' is 'n'.
'sister' with letters rearranged gives 'iserts'.
'n' inserted within 'iserts' is 'INSERTS'.
(Other definitions for inserts that I've seen before include "Incorporates (into something else)" , "Puts in between other things" , "Leaflets in newspapers" .)