Sister nobody’s spoken of (3)
I believe the answer is:
nun
'sister' is the definition.
(sister is a term of address for a nun)
'nobody's spoken of' is the wordplay.
'nobody' becomes 'none' (I've seen this before).
'spoken of' indicates a 'sounds like' (homophone) clue.
'none' sounds like 'NUN'.
(Other definitions for nun that I've seen before include "Fancy pigeon variety" , "Woman in Holy Orders" , "Devout woman" , "A palindromic lady in religious order" , "Mother Teresa, maybe" .)