The most common and most likely answer to this crossword clue is the 8 letter word REACTORS
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More clues leading to the result REACTORS
Buddy explains!

The straight part
We know that "they are sources of energy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they are sources of energy = reactors.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 REACTORS mean?
REACTORS noun- A person who responds to a suggestion, stimulation or other influence.
- (industrial) A structure used to contain chemical or other reactions.
- A device which uses atomic energy to produce heat.
- A chemical substance which responds to the presence or contact with another substance.