The most common and most likely answer to this crossword clue is the 10 letter word INSOLATION
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The straight part
We know that "radiation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case radiation = insolation.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 INSOLATION mean?
INSOLATION noun- The incident radiant energy emitted by the sun which reaches a unit area over a period of time, typically measured over a horizontal area at the Earth's surface or at the top of Earth's atmosphere.
- The rate of delivery of such radiation.
- The act or process of exposing to the rays of the sun, such as for the purpose of medical treatment, drying or maturing, as fruits, drugs, etc., or of rendering acid, as vinegar.
- Sunstroke.