Rocky Marciano consumes energy drink (9)
I believe the answer is:
americano
'drink' is the definition.
(type of coffee drink)
'rocky marciano consumes energy' is the wordplay.
'rocky' is an anagram indicator (letters rocked or thrown around).
'consumes' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'marciano' is an anagram of 'amricano'.
'amricano' going around 'e' is 'AMERICANO'.
(Other definitions for americano that I've seen before include "drink" , "Style of coffee" , "Espresso diluted with hot water" .)