Threatening nun outside home (8)
I believe the answer is:
sinister
'threatening' is the definition.
(I know that threatening can be written as sinister)
'nun outside home' is the wordplay.
'nun' becomes 'sister' (Sister is an example).
'outside' means one lot of letters goes inside another.
'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 "Suggestive of evil, threatening harm" , "Disturbing" , "'Ominous, baleful (8)'" , "not right!" , "Suggestive of evil, to the left in heraldry" .)