Heroin taken regularly for a long time (3)
I believe the answer is:
eon
'long time' is the definition.
(eon is a kind of long time)
'heroin taken regularly' is the wordplay.
'taken regularly' means one should take alternating letters (letters removed at regular intervals).
The alternating letters of 'heroin' are 'EON'.
'for a' is the link.
(Other definitions for eon that I've seen before include "Very long period (US and technical spelling)" , "Vast age (alt. sp.)" , "thousand million years" , "Very long age" , "plenty of time" .)