Tango is an energy drink (6)
I believe the answer is:
tisane
'drink' is the definition.
(tisane is a kind of drink)
'tango is an energy' is the wordplay.
'tango' becomes 'T' (phonetic alphabet: alpha, bravo, charlie etc.).
'energy' becomes 'e' (physics symbol as in E=mc2).
't'+'is'+'an'+'e'='TISANE'
(Other definitions for tisane that I've seen before include "Herbal infusion" , "Herb tea" , "Infusion of e.g. dried or fresh flowers or leaves" , "Herbal tea drink" , "Herb-tea" .)