The most common and most likely answer to this crossword clue is the 10 letter word PERIWINKLE
I've seen this crossword clue in The Guardian
P
E
R
I
W
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K
L
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More clues leading to the result PERIWINKLE
Buddy explains!

The straight part
We know that "light blue" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case light blue = periwinkle.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 PERIWINKLE mean?
PERIWINKLE noun- Any of several evergreen plants of the genus Vinca with blue or white flowers.
- Similar plants of genus Catharanthus.
- A color with bluish and purplish hues, somewhat light.
- Of pale bluish purple colour.
- A mollusk of genus Littorina.