The most common and most likely answer to this crossword clue is the 3 plus 5 plus 2 plus 5 letter word THE SPEED OF LIGHT
I've seen this crossword clue in The Guardian
T
H
E
S
P
E
E
D
O
F
L
I
G
H
T
Buddy explains!
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.