The most common and most likely answer to this crossword clue is the 8 letter word EMISSIVE
I've seen this crossword clue in The Times
E
M
I
S
S
I
V
E
More clues leading to the result EMISSIVE
Buddy explains!

The straight part
We know that "Radiant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case radiant = emissive.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 EMISSIVE mean?
EMISSIVE adjective- Of, pertaining to, or having the capacity to emit radiation or matter; emitting