God, you must stop heroin before the end! (5)
I believe the answer is:
horus
'god' is the definition.
(Egyptian god)
'you must stop heroin before the end' is the wordplay.
'you' becomes 'u' (commonly used in online communication).
'must stop' indicates putting letters inside (stopping can mean blocking a gap).
'heroin' becomes 'horse' (slang term for heroin).
'before the end' means to remove the last letter (all letters before the one at the end).
'horse' with its final letter taken off is 'hors'.
'u' put within 'hors' is 'HORUS'.
(Other definitions for horus that I've seen before include "Falcon-headed god of ancient Egypt" , "Egyptian hawk-headed god" , "Falcon-headed old Egyptian god" , "Deity" , "Egyptian sun-god" .)