Reversal of drug's discontinuance (4)
I believe the answer is:
stop
'discontinuance' is the definition.
(thesaurus)
'reversal of drug's' is the wordplay.
'reversal of' says the letters should be written in reverse.
'drug's' becomes 'pot's' (slang term for cannabis).
'pots' back-to-front is 'STOP'.
(Other definitions for stop that I've seen before include "Discontinue (an action)" , "cut out" , "Cease; organ voice" , "Withhold payment" , "Put an end to" .)