The most common and most likely answer to this crossword clue is the 10 letter word MILLENNIUM
I've seen this crossword clue in Independent
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More clues leading to the result MILLENNIUM
Buddy explains!

The straight part
We know that "many years" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case many years = millennium.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 MILLENNIUM mean?
MILLENNIUM noun- A period of time consisting of one thousand years.
- The period of one thousand years during which Christ will reign on earth (according to Millenarianist interpretations).
- A period of universal happiness, peace or prosperity; a utopia.
- (with definite article) The year in which one period of one thousand years ends and another begins, especially the year 2000.