The most common and most likely answer to this crossword clue is the 2 plus 3 plus 4 plus 1 plus 5 letter word GO OUT LIKE A LIGHT
I've seen this crossword clue in The Guardian
G
O
O
U
T
L
I
K
E
A
L
I
G
H
T
Buddy explains!
The straight part
We know that "soon lose consciousness" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case soon lose consciousness = go out like a light.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.