Routine amphetamine overdose (6)
I believe the answer is:
method
'routine' is the definition.
(thesaurus)
'amphetamine overdose' is the wordplay.
'amphetamine' becomes 'meth' (meth is a kind of amphetamine).
'overdose' becomes 'o.d.' (synonyms).
'meth'+'od'='METHOD'
(Other definitions for method that I've seen before include "Process, esp. Stanislavsky's" , "'Means, way of achieving (6)'" , "Orderliness; procedure" , "Manner, technique" , "Way of doing something, technique" .)