Threatening relative outside home (8)
I believe the answer is:
sinister
'threatening' is the definition.
(synonyms)
'relative outside home' is the wordplay.
'relative' becomes 'sister' (sister is a kind of relative).
'outside' means one lot of letters goes inside another.
'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 "Suggestive of evil, being lefthanded" , "What is left on bearings" , "Threatening evil on the heraldic left" , "can be injurious" , "Menacing" .)