The most common and most likely answer to this crossword clue is the 7 plus 5 letter word ETERNAL FLAME
I've seen this crossword clue in The Guardian
E
T
E
R
N
A
L
F
L
A
M
E
Buddy explains!
The straight part
We know that "song (There Is A Light That Never Goes Out)" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case song (there is a light that never goes out) = eternal flame.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.