Sister not a soul heard (3)
I believe the answer is:
nun
'sister' is the definition.
(sister is a term of address for a nun)
'not a soul heard' is the wordplay.
'not a soul' becomes 'none' (both can mean nobody).
'heard' shows a homophone (sound like).
'none' sounds like 'NUN'.
(Other definitions for nun that I've seen before include "One who's taken vows" , "Mother Superior, perhaps" , "Woman respecting order" , "She's a member of a religious community" , "Member of religious order" .)