No sister, reportedly (4)
I believe the answer is:
none
'no' is the definition.
(I've seen this in another clue)
'sister reportedly' is the wordplay.
'sister' becomes 'nun' (sister is a term of address for a nun).
'reportedly' indicates a 'sounds like' (homophone) clue.
'nun' sounds like 'NONE'.
(Other definitions for none that I've seen before include "Not even one" , "hour of prayer" , "service for monks" , "office" , "0?" .)