The most common and most likely answer to this crossword clue is the 9 letter word RADIOLOGY
I've seen this crossword clue in The Guardian
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More clues leading to the result RADIOLOGY
Buddy explains!
The straight part
We know that "cancer therapy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cancer therapy = radiology.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 RADIOLOGY mean?
RADIOLOGY noun- The use of radioactive substances in diagnosis and treatment.
- The use of radiation (including ionizing radiation, especially X-rays) in diagnosis, usually through the formation of images.
- Radioscopy.