Problem doing without heroin for long (4)
I believe the answer is:
itch
'long' is the definition.
(to long or itch for something is to desire it)
'problem doing without heroin' is the wordplay.
'problem' becomes 'hitch' (I've seen this before).
'doing without' indicates named letters should be taken away.
'heroin' becomes 'H' (synonyms).
'hitch' with 'h' taken away is 'ITCH'.
'for' is the link.
(Other definitions for itch that I've seen before include "Prickle" , "Irritate" , "Tickling sensation of skin" , "Skin irritation" , "that's irritating" .)