Root used in energy drinks (7)
I believe the answer is:
ginseng
'root used in energy drinks' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ginseng that I've seen before include "Root believed to have curative properties" , "Medicinal tonic root" , "Chinese tonic" , "Plant drug" , "Root used in Chinese medicine" .)