Killer terrier runs out (4)
I believe the answer is:
cain
'killer' is the definition.
(Cain who killed Abel)
'terrier runs out' is the wordplay.
'terrier' becomes 'cairn terrier' (cairn is a kind of terrier).
'runs' becomes 'r' (cricket abbreviation).
'out' is a deletion indicator.
'cairn' with 'r' taken away is 'CAIN'.
(Other definitions for cain that I've seen before include "Brother of Abel" , "First son" , "Abel's brother" , "Eve's firstborn" , "Adam's son" .)