Relative holding in evil (8)
I believe the answer is:
sinister
'evil' is the definition.
(similar in meaning)
'relative holding in' is the wordplay.
'relative' becomes 'sister' (sister is a kind of relative).
'holding' means one lot of letters goes inside another.
'sister' enclosing 'in' is 'SINISTER'.
(Other definitions for sinister that I've seen before include "can be injurious" , "On the left (of shield) (her.); wicked, criminal" , ", nun going round home" , "Suggestive of evil, being lefthanded" , "Forbidding" .)