Forbidding nun to ring home (8)
I believe the answer is:
sinister
'forbidding' is the definition.
('sinister' can be a synonym of 'forbidding')
'nun to ring home' is the wordplay.
'nun' becomes 'sister' (Sister is an example).
'to ring' is an insertion indicator.
'home' becomes 'in' ('he's home' can mean 'he's in').
'sister' enclosing 'in' is 'SINISTER'.
(Other definitions for sinister that I've seen before include "Forbidding" , "Menacing" , "Spooky" , "Threatening evil on the heraldic left" , "Suggesting or threatening evil or danger" .)