The killer was following two donkeys in (8)
I believe the answer is:
assassin
'the killer' is the definition.
(assassin is a kind of killer)
'following two donkeys in' is the wordplay.
'two donkeys' means two replacements for 'donkey'.
'following' means one lot of letters go next to another.
'donkey' becomes 'ass' (ass is a term for a donkey).
'donkey' becomes 'ass' (ass is a term for a donkey).
'ass' put next to 'ass' is 'assass'.
'assass'+'in'='ASSASSIN'
'was' acts as a link.
(Other definitions for assassin that I've seen before include "One murders for political or monetary reasons" , "Killer of famous person" , "One making hits" , "Hitman" , "I kill" .)