Previously left at home in care of nurse (8)
I believe the answer is:
'previously left' is the definition.
(sinister could historically mean left)
'home in care of nurse' is the wordplay.
'home' becomes 'in' (to be in is to be (at) home).
'in care of' means one lot of letters goes inside another.
'nurse' becomes 'sister' (I've seen this before).
'in' placed inside 'sister' is 'SINISTER'.
'at' is the link.
(Other definitions for sinister that I've seen before include "Disquieting" , "Menacing" , "Suggesting or threatening evil or danger" , "Suggestive of evil, to the left in heraldry" , "Threatening evil on the heraldic left" .)