Nun's kept in the dark? (8)
I believe the answer is:
sinister
'the dark?' is the definition.
(thesaurus)
'nun's kept in' is the wordplay.
'nun' becomes 'sister' (Sister is an example).
'kept' is an insertion indicator.
'sister' enclosing 'in' is 'SINISTER'.
(Other definitions for sinister that I've seen before include "Suggestive of evil, to the left in heraldry" , "Suggestive of evil, being lefthanded" , "can be injurious" , "Resist in way that is creepy" , "Menacing" .)