Causing death is allowed by 2001 computer (6)
I believe the answer is:
lethal
'causing death' is the definition.
(I've seen this before)
'allowed by 2001 computer' is the wordplay.
'allowed' becomes 'let' (I've seen this before).
'by' means one lot of letters go next to another.
'2001 computer' becomes 'hal' (HAL 9000 is a computer in 2001: A Space Odyssey).
'let'+'hal'='LETHAL'
'is' acts as a link.
(Other definitions for lethal that I've seen before include "Capable of causing death" , "Death-dealing" , "bringing down the curtain" , "Destructive" , "bringing about the end?" .)