Energy drink gets youth initially nervous (4)
I believe the answer is:
edgy
'nervous' is the definition.
('edgy' can be a synonym of 'nervous')
'energy drink gets youth initially' is the wordplay.
'initially' suggests taking the first letters.
The initial letters of 'energy drink gets youth' is 'EDGY'.
(Other definitions for edgy that I've seen before include "Visibly nervous" , "Nervous and tense, irritable" , "Keyed up" , "In a tense state" , "Sharp in outline" .)