Not a single sister is heard (4)
I believe the answer is:
none
'not a single' is the definition.
(not one)
'sister is heard' is the wordplay.
'sister' becomes 'nun' (sister is a term of address for a nun).
'is heard' indicates a 'sounds like' (homophone) clue.
'nun' sounds like 'NONE'.
(Other definitions for none that I've seen before include "Not a sausage" , "Not a soul" , "Zero" , "0?" , "service for monks" .)