News about union leader’s sister (3)
I believe the answer is:
nun
'sister' is the definition.
(thesaurus)
'news about union leader's' is the wordplay.
'news' becomes 'nn' (I've seen this in other clues).
'about' means one lot of letters goes inside another.
'leader's' suggests taking the first letters (I've seen 'leaders' mean this).
The initial letter of 'union' is 'u'.
'nn' going around 'u' is 'NUN'.
(Other definitions for nun that I've seen before include "Woman respecting order" , "Member of religious order" , "Woman in Holy Orders" , "A palindromic lady in religious order" , "Black and white pigeon" .)