Heroin rather leads to addiction (5)
I believe the answer is:
habit
'addiction' is the definition.
(thesaurus)
'heroin rather' is the wordplay.
'heroin' becomes 'h' ('H' can be a synonym of 'heroin').
'rather' becomes 'abit' (I've seen this before).
'h'+'abit'='HABIT'
'leads to' acts as a link.
(Other definitions for habit that I've seen before include "Routine - clothing" , "Custom, nun's wear" , "Monk's robe or repetitive action" , "addictive practice" , "Custom; riding dress" .)