Habit-forming (9)
I believe the answer is:
addictive
'habit-forming' is the definition.
(something addictive forms a habit)
This is the entire clue.
(Other definitions for addictive that I've seen before include "...that can't be ignored" , "Hard to give up" , "Cite David for causing compulsive or habit-forming behaviour" , "Maybe like harmful substance" , "Gushing" .)