Berserk (4)
I believe the answer is:
amok
'berserk' is the definition.
(synonyms)
This is all the clue.
(Other definitions for amok that I've seen before include "(Run) in murderous frenzy" , "Going mad" , "Run thus to run wildly about" , "(Run) in frenzied state" , "In an uncontrolled, often violent, way" .)