Tim leaves Casualty with drug habit (4)
I believe the answer is:
vice
'habit' is the definition.
(I have seen 'Bad habit' mean 'vice' so perhaps 'habit' could also mean 'vice')
'tim leaves casualty with drug' is the wordplay.
'leaves' suggests deleting specific letters.
'casualty' becomes 'victim' (both can mean someone injured or killed).
'with' is a charade indicator (letters next to each other).
'drug' becomes 'e' (E is another name for ecstasy).
'victim' with 'tim' taken out is 'vic'.
'vic'+'e'='VICE'
(Other definitions for vice that I've seen before include "In place of - depravity" , "Sin; weakness" , "Immorality; clamping device" , "item in workshop" , "Tool with gripping jaws" .)