The most common and most likely answer to this crossword clue is the 10 letter word ANTITHESIS
I've seen this crossword clue in The Daily Telegraph
A
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More clues leading to the result ANTITHESIS
- Soldier by river cases the opposition
- An idiot is stifling man’s opposition
- Essay ain’t bad to start with? The opposite
- Is the Saint perplexed? Quite the opposite!
- Slaughter is hesitant in opposition
- The saint wicked? Is the opposite
- Contrary contribution to Christian tithes I subscribe
- A new tax meeting relative opposition
- Troubled, as it is then quite the contrary?
- Worker has it, he is on island opposite
Buddy explains!

The straight part
We know that "the opposite" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case the opposite = antithesis.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 ANTITHESIS mean?
ANTITHESIS noun- A proposition that is the diametric opposite of some other proposition.
- A device by which two contrasting ideas are juxtaposed in parallel form.