The most common and most likely answer to this crossword clue is the 9 plus 6 letter word RENEWABLE ENERGY
I've seen this crossword clue in The Daily Telegraph
R
E
N
E
W
A
B
L
E
E
N
E
R
G
Y
More clues leading to the result RENEWABLE ENERGY
Buddy explains!
The straight part
We know that "Alternative sources of power" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case alternative sources of power = renewable energy.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.