Left at home to occupy a sibling (8)
I believe the answer is:
sinister
'left' is the definition.
(I've seen this before)
'home to occupy a sibling' is the wordplay.
'home' becomes 'in' ('he's home' can mean 'he's in').
'to occupy' means one lot of letters goes inside another (inserted letters occupy or enter the word).
'a sibling' becomes 'sister' (female sibling).
'in' put within 'sister' is 'SINISTER'.
'at' acts as a link.
(Other definitions for sinister that I've seen before include "On the left (of shield) (her.); wicked, criminal" , "Disquieting" , "Forbidding" , "Very unpleasant" , "Suggestive of evil, threatening harm" .)