Heroin in crack causes pain (4)
I believe the answer is:
ache
'pain' is the definition.
(ache is a kind of pain)
'heroin in crack' is the wordplay.
'heroin' becomes 'h' ('H' can be a synonym of 'heroin').
'in' indicates putting letters inside.
'crack' becomes 'ace' (I've seen this before).
'h' inserted inside 'ace' is 'ACHE'.
'causes' acts as a link.
(Other definitions for ache that I've seen before include "Hurt; yearn" , "Play up" , "Cause pain" , "Twinge" , "Feel sore" .)