The most common and most likely answer to this crossword clue is the 13 plus 5 letter word ELECTROMOTIVE FORCE
I've seen this crossword clue in The Times
E
L
E
C
T
R
O
M
O
T
I
V
E
F
O
R
C
E
Buddy explains!
The straight part
We know that "What wave power generates" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what wave power generates = electromotive force.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.