The most common and most likely answer to this crossword clue is the 12 letter word CARBOHYDRATE
I've seen this crossword clue in The Guardian
C
A
R
B
O
H
Y
D
R
A
T
E
Buddy explains!

The straight part
We know that "energy source" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case energy source = carbohydrate.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does CARBOHYDRATE mean?
CARBOHYDRATE noun- (nutrition) A sugar, starch, or cellulose that is a food source of energy for an animal or plant.
- (by extension, metonym) Any food rich in starch or other carbohydrates.