The most common and most likely answer to this crossword clue is the 10 letter word FLOODLIGHT
I've seen this crossword clue in Financial Times
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Buddy explains!
The straight part
We know that "source of brightness" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case source of brightness = floodlight.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 FLOODLIGHT mean?
FLOODLIGHT noun- A projector of a bright beam of light for use in theatres and studios; a flood
- (chiefly in the plural) Powerful artificial illumination with a broad beam, especially in a series of units on pylons used to illuminate a sports ground.
- To enlighten or illuminate with floodlight(s).