Drug’s given over bar (4)
I believe the answer is:
stop
'bar' is the definition.
(bar can mean to stop or prevent)
'drug's given over' is the wordplay.
'drug's' becomes 'pot's' (slang term for cannabis).
'given over' says the letters should be written in reverse.
'pots' backwards is 'STOP'.
(Other definitions for stop that I've seen before include "period in US" , "Red light signal" , "Cease, halt" , "Withhold payment" , "Check" .)