Custom somewhat hard at first (5)
I believe the answer is:
habit
'custom' is the definition.
(habit is a kind of custom)
'somewhat hard at first' is the wordplay.
'somewhat' becomes 'abit' (a bit).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'at first' means one lot of letters go next to another.
'abit' put after 'h' is 'HABIT'.
(Other definitions for habit that I've seen before include "Ingrained behaviour" , "Monk's garb" , "Regular practice" , "can cause addiction" , "Custom; riding dress" .)