The most common and most likely answer to this crossword clue is the 10 letter word PENSIONERS
I've seen this crossword clue in Financial Times
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Buddy explains!
The straight part
We know that "oldsters" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case oldsters = pensioners.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 PENSIONERS mean?
PENSIONERS noun- Someone who lives on a pension, especially the retirement or old age pension.
- (by extension) Someone who is at the age at which one typically receives a pension; an elderly person.
- (Cambridge University) A student who is not dependent on any foundation for support, but pays all university charges; at Oxford called a commoner.