Unmarried sister (3)
I believe the answer is:
nun
This clue is a double definition.
'unmarried' is the first definition.
(thesaurus)
'sister' is the second definition.
(sister is a term of address for a nun)
(Other definitions for nun that I've seen before include "A palindromic lady in religious order" , "Fancy pigeon variety" , "One who's taken vows" , "Devout woman" , "eg Carmelite" .)