The most common and most likely answer to this crossword clue is the 10 letter word BRIGHTNESS
I've seen this crossword clue in The Daily Telegraph
B
R
I
G
H
T
N
E
S
S
More clues leading to the result BRIGHTNESS
Buddy explains!

The straight part
We know that "Radiance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case radiance = brightness.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.
What does BRIGHTNESS mean?
BRIGHTNESS noun- The quality of being bright.
- The perceived luminance of an object.
- Intelligence, cleverness.