Drug habit in sport (8)
I believe the answer is:
speedway
'sport' is the definition.
(I've seen this before)
'drug habit' is the wordplay.
'drug' becomes 'speed' (speed is a kind of drug).
'habit' becomes 'way' (habit or way of doing something).
'speed'+'way'='SPEEDWAY'
'in' is the link.
(Other definitions for speedway that I've seen before include "Dirt track motorcycle sport" , "Dirt track motorcycle racing" , "Form of motorcycle racing" , "fast-track competition" , "Stadium for motorcycle racing" .)