The most common and most likely answer to this crossword clue is the 4 letter word YUGA
Y
U
G
A
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The straight part
We know that "a long time" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a long time = yuga.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 YUGA mean?
YUGA noun- In Hindu theology, an epoch or era within a cycle of four ages: the Satya Yuga (or Krita Yuga), the Dvapara Yuga, the Treta Yuga and finally the Kali Yuga, with lengths ranging from 432,000 to 1,728,000 years.