The most common and most likely answer to this crossword clue is the 8 letter word REMAINER
I've seen this crossword clue in The Times
R
E
M
A
I
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Buddy explains!
The straight part
We know that "One opposed to leaving" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one opposed to leaving = remainer.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 REMAINER mean?
REMAINER noun- (UK politics) Someone who supports the United Kingdom remaining within the European Union.
- One who remains.
- A part or parts remaining after some has/have been removed.
- The amount left over after subtracting the divisor as many times as possible from the dividend without producing a negative result. If (n) (dividend) and d (divisor) are integers, then (n) can always be expressed in the form n = dq + r, where q (quotient) and r (remainder) are also integers and 0 ≤ r < d.
- The number left over after a simple subtraction
- Excess stock items left unsold and subject to reduction in price.
- An estate in expectancy which only comes in its heir's possession after an estate created by the same instrument has been determined