Nun holding in evil (8)
I believe the answer is:
sinister
'evil' is the definition.
(similar in meaning)
'nun holding in' is the wordplay.
'nun' becomes 'sister' (Sister is an example).
'holding' is an insertion indicator.
'sister' going around 'in' is 'SINISTER'.
(Other definitions for sinister that I've seen before include "On the left (of shield) (her.); wicked, criminal" , "Threatening; evil-looking" , "Threatening the left in heraldry" , "Forbidding" , "Menacing" .)