Menacing relative outside home (8)
I believe the answer is:
sinister
'menacing' is the definition.
('sinister' can be a synonym of 'menacing')
'relative outside home' is the wordplay.
'relative' becomes 'sister' (sister is a kind of relative).
'outside' is an insertion indicator.
'home' becomes 'in' ('he's home' can mean 'he's in').
'sister' placed around 'in' is 'SINISTER'.
(Other definitions for sinister that I've seen before include "Suggestive of evil, to the left in heraldry" , "Very unpleasant" , "Resist in way that is creepy" , "Disturbing" , "Eerie, alarming" .)