Some addicts kick this rather hard at first (5)
I believe the answer is:
habit
'some addicts kick this' is the definition.
(to 'kick the habit' could be to give up drugs)
'rather hard at first' is the wordplay.
'rather' becomes 'abit' (I've seen this before).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'at first' says to put letters next to each other.
'abit' put after 'h' is 'HABIT'.
(Other definitions for habit that I've seen before include "Monk's gown or custom" , "It's worn in order" , "Routine - clothing" , "What might one have a regular tendency to" , "Custom; riding dress" .)