The most common and most likely answer to this crossword clue is the 11 letter word FLUORESCENT
I've seen this crossword clue in The Sunday Telegraph
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Buddy explains!
The straight part
We know that "Giving off glow" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case giving off glow = fluorescent.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 FLUORESCENT mean?
FLUORESCENT noun- A fluorescent light.
- Of or relating to fluorescence.
- Exhibiting or produced by fluorescence.
- Emitting visible light as a result of the excitation of phosphors by ultraviolet photons produced by the passage of an electrical current through an inert gas infused with mercury.
- Glowing as if with fluorescence; vivid