The most common and most likely answer to this crossword clue is the 9 letter word STERADIAN
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More clues leading to the result STERADIAN
Buddy explains!
The straight part
We know that "unit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unit = steradian.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 STERADIAN mean?
STERADIAN noun- In the International System of Units, the derived unit of solid angle; the solid angle subtended at the centre of a sphere of radius r by a portion of the surface of the sphere that has area r2. Symbol: sr