An excessive and dangerous amount of a drug (8)
I believe the answer is:
overdose
'an excessive and dangerous amount of a drug' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for overdose that I've seen before include "Excessive amount (of drugs)" , "Take too much of a drug" , "Too many pills" , "Excessive intake of drug" , "Drug excess" .)