The most common and most likely answer to this crossword clue is the 5 letter word ENEMY
I've seen this crossword clue in The Guardian
E
N
E
M
Y
More clues leading to the result ENEMY
- You round soldiers up as an adversary
- In revolution you must capture soldiers who will want to kill you?
- Opposing troops some cavalrymen eventually pushed back
- Soldiers captured by you no longer turned up as opposition
- 8 soldiers stopping the old adversary
- Two letters in print seen by unknown adversary
- People in revolution breaking heart of fogeyish opponent
- Opponent of energy with which young chaps go west
- Pennies oddly lacking – the writer’s nemesis
- Evacuation: one leaves before last of Tory Opposition
Buddy explains!

The straight part
We know that "Opponent" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case opponent = enemy.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 ENEMY mean?
ENEMY noun- Someone who is hostile to, feels hatred towards, opposes the interests of, or intends injury to someone else.
- A hostile force or nation; a fighting member of such a force or nation.
- Something harmful or threatening to another
- To make an enemy of.
- Of, relating to, or belonging to an enemy