The most common and most likely answer to this crossword clue is the 7 letter word BIOMASS
B
I
O
M
A
S
S
More clues leading to the result BIOMASS
- Fraud is a mob’s source of power
- Iodine or mercury initially found in fish used as organic fuel
- Organic source of energy from fish circling island
- Man tucked into fish, possible source of energy
- Fish around Isle of Man may be source of energy
- Amount of life as mob is unruly
- Black donkey seen around British isle with a whole lot of plants and animals
- Buy, say, fried samosa — for the most part. It gives energy
Buddy explains!

The straight part
We know that "possible source of energy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case possible source of energy = biomass.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 BIOMASS mean?
BIOMASS noun- The total mass of a living thing or part thereof, such as a cell.
- The total mass of all living things within a specific area, habitat etc.
- Vegetation used as a fuel, or source of energy, especially if cultivated for that purpose.