Setter ingesting ecstasy and heroin got nasty bug (8)
I believe the answer is:
pathogen
'bug' is the definition.
(thesaurus)
'setter ingesting ecstasy and heroin got nasty' is the wordplay.
I cannot quite see how this works, but
'ecstasy' could be 'e' (informal term for the drug) and 'e' is present in the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is found within the answer.
'heroin' could be 'h' (synonyms) and 'h' is located in the answer.
an anagram of 'got' is 'tog' which is located in the remaining letters.
The remaining letters 'pa' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
(Other definitions for pathogen that I've seen before include "it may make one sick" , "Disease-causing agent" , "Heptagon (anag.)" , "Disease-causing micro-organism" , "Cause of disease" .)