The most common and most likely answer to this crossword clue is the 10 letter word CONTROVERT
I've seen this crossword clue in The Daily Telegraph
C
O
N
T
R
O
V
E
R
T
More clues leading to the result CONTROVERT
Buddy explains!

The straight part
We know that "Argue against" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case argue against = controvert.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 CONTROVERT mean?
CONTROVERT verb- To dispute, to argue about (something).
- To argue against (something or someone); to contradict, to deny.
- To be involved or engaged in controversy; to argue.