Monster dog ran amok (6)
I believe the answer is:
dragon
'monster' is the definition.
(dragon is a kind of monster)
'dog ran amok' is the wordplay.
'amok' indicates an anagram.
'dog'+'ran'='dogran'
'dogran' anagrammed gives 'DRAGON'.
(Other definitions for dragon that I've seen before include "St George slew one" , "St George is said to have killed one" , "Creature of Teutonic mythology" , "Fire breathing mythological creature" , "Teutonic mythological creature" .)