Killer insect wife brushed off (3)
I believe the answer is:
asp
'killer' is the definition.
(Kind of poisonous snake)
'insect wife brushed off' is the wordplay.
'insect' becomes 'wasp' (wasp is a kind of insect).
'wife' becomes 'w' (abbreviation).
'brushed off' is a deletion indicator.
'wasp' with 'w' taken out is 'ASP'.
(Other definitions for asp that I've seen before include "Cleopatra's supposed nemesis" , "Cleopatra-killing snake" , "One secreting poison" , "This Egyptian killer" , "Snake of the Nile" .)