More furtive adult missed by violent killer (5)
I believe the answer is:
slyer
'more furtive' is the definition.
(furtive means sly or secret)
'adult missed by violent killer' is the wordplay.
'adult' becomes 'a' (abbreviation).
'missed by' suggests deleting specific letters.
'violent killer' becomes 'slayer' (someone who slays).
'slayer' with 'a' taken out is 'SLYER'.
(Other definitions for slyer that I've seen before include "More cunning" , "more sneaky" .)