The most common and most likely answer to this crossword clue is the 8 letter word CONFRONT
I've seen this crossword clue in The Daily Telegraph
C
O
N
F
R
O
N
T
More clues leading to the result CONFRONT
Buddy explains!

The straight part
We know that "Oppose" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case oppose = confront.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 CONFRONT mean?
CONFRONT verb- To stand or meet facing, especially in competition, hostility or defiance; to come face to face with
- To deal with.
- To something bring face to face with.
- To come up against; to encounter.
- To engage in confrontation.
- To set a thing side by side with; to compare.
- To put a thing facing to; to set in contrast to.