Sources of energy drink get youth nervous (4)
I believe the answer is:
edgy
'nervous' is the definition.
('edgy' can be a synonym of 'nervous')
'sources of energy drink get youth' is the wordplay.
'sources of' indicates taking the first letters.
The initial letters of 'energy drink get youth' is 'EDGY'.
(Other definitions for edgy that I've seen before include "Apprehensive" , "Keyed up" , "Nervous and tense, irritable" , "Jittery" , "Overstrung" .)