Heroin used regularly for a long time (3)
I believe the answer is:
eon
'long time' is the definition.
(I know that eon is a type of long time)
'heroin used regularly' is the wordplay.
'used regularly' means one should take alternating letters.
The alternating letters of 'heroin' are 'EON'.
'for a' is the link.
(Other definitions for eon that I've seen before include "An immense time" , "Immense period" , "thousand million years" , "Huge time period" , "Vast age (alt. sp.)" .)