Heroin to some extent creates addiction (5)
I believe the answer is:
habit
'addiction' is the definition.
(I've seen this before)
'heroin to some extent' is the wordplay.
'heroin' becomes 'H' (H is a slang term for heroin).
'to some extent' becomes 'a bit' (to a small extent).
'h'+'abit'='HABIT'
'creates' acts as a link.
(Other definitions for habit that I've seen before include "Monk's gown or custom" , "Breaking this is hard" , "bent" , "Ingrained behaviour" , "Custom; riding dress" .)