The most common and most likely answer to this crossword clue is the 10 letter word LIGHT-YEAR
I've seen this crossword clue in Financial Times
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More clues leading to the result LIGHT-YEAR
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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 LIGHT-YEAR mean?
LIGHT-YEAR noun- A unit of length (abbreviation ly; equal to just under 10 trillion kilometres (1016 metres)) equal to the distance light travels in one Julian year; used to measure extremely large distances.
- (only in plural) A very long way.
- (only in plural) A very long time.