Old tin containing energy drink (5)
I believe the answer is:
ocean
'drink' is the definition.
('the drink' is an informal term for the sea)
'old tin containing energy' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'tin' becomes 'can' ('can' can be a synonym of 'tin').
'containing' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mc2).
'can' going around 'e' is 'cean'.
'o'+'cean'='OCEAN'
(Other definitions for ocean that I've seen before include "A mighty sea" , "Vast sea" , "Indian or Arctic, say" , "The deep" , "Extensive sea area" .)