Stop drug that tackles nothing (7)
I believe the answer is:
'stop' is the definition.
(eg a stop on a train journey)
'drug that tackles nothing' is the wordplay.
'drug' becomes 'statin' (type of drug).
'that tackles' indicates putting letters inside (some letters tackle or block in others).
'nothing' becomes 'O' (looks like zero - 0).
'statin' enclosing 'o' is 'STATION'.
(Other definitions for station that I've seen before include "Train halt" , "Depot" , "TV or radio channel" , "Chosen fixed point" , "Place of duty, work" .)