Cunning, unpleasant son knocks out Victor (9)
I believe the answer is:
insidious
'cunning' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for insidious that I've seen before include "Treacherously sly" , "Cunning" , "crafty" , "Treacherous; subtle" , "Stealthy and damaging" .)