The most common and most likely answer to this crossword clue is the 5 letter word YEARS
I've seen this crossword clue in The Guardian
Y
E
A
R
S
More clues leading to the result YEARS
- At least twenty-four months is a long time
- Certainly welcoming retired artist for a long time
- Unreliable hearsay a hospital dismissed for ages
- Age of maturity finally acquired by organs
- Youth leader ’appily listens a long time
- Extremely restless after vote for a long time.
- Positive response protects a source of revenue for ages
- Pays attention to not hard but extremely easy periods
- Longs, giving up some nourishment, for long periods
- Pines when knight’s away for long stretches of time
Buddy explains!

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 = years.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 YEARS mean?
YEARS noun- A solar year, the time it takes the Earth to complete one revolution of the Sun (between 365.24 and 365.26 days depending on the point of reference).
- (by extension) The time it takes for any astronomical object (such as a planet, dwarf planet, small Solar System body, or comet) in direct orbit around a star (such as the Sun) to make one revolution around the star.
- A period between set dates that mark a year, from January 1 to December 31 by the Gregorian calendar, from Tishiri 1 to Elul 29 by the Jewish calendar, and from Muharram 1 to Dhu al-Hijjah 29 or 30 by the Islamic calendar.
- A scheduled part of a calendar year spent in a specific activity.
- A Julian year, exactly 365.25 days, represented by "a".
- A level or grade in school or college.
- The proportion of a creature's lifespan equivalent to one year of an average human lifespan (see also dog year).
- A very long time.