Routine drug overdose (6)
I believe the answer is:
method
'routine' is the definition.
(thesaurus)
'drug overdose' is the wordplay.
'drug' becomes 'meth' (meth is a kind of drug).
'overdose' becomes 'od' (synonyms).
'meth'+'od'='METHOD'
(Other definitions for method that I've seen before include "particular procedure" , "Procedure or system" , "Systematic procedure" , "Type of acting" , "Way of doing something, even in madness (6)" .)