Addiction is slightly hard at first (5)
I believe the answer is:
habit
'addiction' is the definition.
(thesaurus)
'slightly hard at first' is the wordplay.
'slightly' becomes 'abit' (I've seen this before).
'hard' becomes 'h'.
'at first' is a charade indicator (letters next to each other).
'abit' put after 'h' is 'HABIT'.
'is' is the link.
(Other definitions for habit that I've seen before include "Acquired pattern of behaviour" , "Monk's robe or repetitive action" , "addictive practice" , "can cause addiction" , "Usual practice; garment" .)