Drunk injects heroin and crack (4)
I believe the answer is:
shot
'crack' is the definition.
(I know that crack can be written as shot)
'drunk injects heroin' is the wordplay.
'drunk' becomes 'sot' (sot can mean a drunken person).
'injects' indicates putting letters inside.
'heroin' becomes 'H' (H is a slang term for heroin).
'sot' enclosing 'h' is 'SHOT'.
'and' acts as a link.
(Other definitions for shot that I've seen before include "Snap" , "Attempt (to kill?)" , "Attempt - hit with bullet" , "Crack" , "Sent a bullet" .)