Cryptic crossword clue
A miser’s apt to become perverse, not joining in 10

The most common and most likely answer to this crossword clue is the 10 letter word SEPARATISM

I've seen this crossword clue in The Guardian
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Buddy explains!

The straight part
We know that "not joining in" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not joining in = separatism.
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 SEPARATISM mean?

SEPARATISM noun
  1. A theory or doctrine which supports a state of separation between organizations, institutions, or other societal groups (e.g. between church and state) or between different political jurisdictions (e.g. a country and its former colony).
  2. The practice of treating members of different societal groups in a politically, legally, or economically different manner.

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