The most common and most likely answer to this crossword clue is the 11 letter word BACKSCATTER
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Buddy explains!

The straight part
We know that "deflection of radiation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case deflection of radiation = backscatter.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 BACKSCATTER mean?
BACKSCATTER noun- The deflection of particles and/or radiation through angles greater than 90 degrees to the original direction of travel.
- The particles and/or radiation deflected in this manner.
- A portion of the energy of electromagnetic radiation such as a laser or radio waves that is scattered back in the direction of the source of radiation by an obscurant.
- The sending of IP packets to essentially random addresses in response to incoming packets that have a spoofed origin.
- To scatter particles and/or radiation back to the direction from which they come.