Sibling eating at home - that's ominous (8)
I believe the answer is:
sinister
'that's ominous' is the definition.
(synonyms)
'sibling eating at home' is the wordplay.
'sibling' becomes 'sister' (I've seen this before).
'eating' indicates putting letters inside.
'at home' becomes 'in' ('I'm in' can mean 'I'm at home').
'sister' going around 'in' is 'SINISTER'.
(Other definitions for sinister that I've seen before include "Menacing" , "not right!" , "Forbidding" , "'Ominous, baleful (8)'" , "Suggesting or threatening evil or danger" .)