Dangerous agent's course of action gone awry (8)
I believe the answer is:
pathogen
'dangerous agent's' is the definition.
'course of action gone awry' is the wordplay.
'course of action' becomes 'path'.
'awry' indicates an anagram.
'gone' is an anagram of 'ogen'.
'path'+'ogen'='PATHOGEN'
(Other definitions for pathogen that I've seen before include "it may make one sick" , "Disease-causing organism" , "It makes us sick" , "source of disease" , "Cause of disease" .)