Drug best known initially to keep dancers going in trance (7)
I believe the answer is:
ecstasy
I believe this is a double definition.
'drug best known initially to keep dancers going' is the first definition.
(drug popular with ravers known by its initial E)
'trance' is the second definition.
(thesaurus)
'in' is the link.
(Other definitions for ecstasy that I've seen before include "Feeling of intense delight, or stimulant drug" , "Extreme happiness" , "Rapture; drug" , "Feeling of immense rapture" , "Intense happiness" .)