Ominous, nun ringing home (8)
I believe the answer is:
sinister
'ominous' is the definition.
('sinister' can be a synonym of 'ominous')
'nun ringing home' is the wordplay.
'nun' becomes 'sister' (Sister is an example).
'ringing' is an insertion indicator.
'home' becomes 'in' ('he's home' can mean 'he's in').
'sister' going around 'in' is 'SINISTER'.
(Other definitions for sinister that I've seen before include "On the left (of shield) (her.); wicked, criminal" , ", nun going round home" , "Menacing" , "Malign, ominous" , "Suggestive of evil, being lefthanded" .)