The most common and most likely answer to this crossword clue is the 3 letter word PEP
I've seen this crossword clue in The Times
P
E
P
More clues leading to the result PEP
Buddy explains!

The straight part
We know that "Energy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case energy = pep.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 PEP mean?
PEP noun- Energy, high spirits.
- To inject with energy and enthusiasm.