Leading energy drink (4)
I believe the answer is:
tope
'drink' is the definition.
('tope' can be a synonym of 'drink')
'leading energy' is the wordplay.
'leading' becomes 'top'.
'energy' becomes 'e' (physics symbol as in E=mc2).
'top'+'e'='TOPE'
(Other definitions for tope that I've seen before include "Drink hard regularly" , "What habitual drinkers do" , "Drink to excess" , "Small grey requiem shark" , "plantation?" .)