Relation dumped outside home? That's ominous (8)
I believe the answer is:
sinister
'ominous' is the definition.
('sinister' can be a synonym of 'ominous')
'relation dumped outside home?' is the wordplay.
'relation' becomes 'sister' (sister is a kind of relation).
'dumped outside' indicates putting letters inside (I've seen this in other clues).
'home?' becomes 'in' ('he's home' can mean 'he's in').
'sister' placed around 'in' is 'SINISTER'.
'that's' acts as a link.
(Other definitions for sinister that I've seen before include "Suggestive of evil, to the left in heraldry" , "Suggestive of evil/harm" , "Eerie, alarming" , "What is left on bearings" , "Menacing" .)