The most common and most likely answer to this crossword clue is the 6 letter word PALEST
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More clues leading to the result PALEST
Buddy explains!

The straight part
We know that "Lightest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lightest = palest.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 PALEST mean?
PALEST verb- To turn pale; to lose colour.
- To become insignificant.
- To make pale; to diminish the brightness of.
- Light in color.
- (of human skin) Having a pallor (a light color, especially due to sickness, shock, fright etc.).
- Feeble, faint.
- To enclose with pales, or as if with pales; to encircle or encompass; to fence off.