Accounts ultimately for energy drinks (7)
I believe the answer is:
reports
'accounts' is the definition.
(I know this)
'ultimately for energy drinks' is the wordplay.
'ultimately' says to take the final letters.
'energy' becomes 'e' (physics symbol as in E=mco).
'drinks' becomes 'ports' (port is a kind of drink).
The last letter of 'for' is 'r'.
'r'+'e'+'ports'='REPORTS'
(Other definitions for reports that I've seen before include "Sounds of gunfire" , "Short accounts of the news" , "Statements" , "Gives account" , "Writes about, booms" .)