The most common and most likely answer to this crossword clue is the 7 letter word RESISTS
I've seen this crossword clue in The Guardian
R
E
S
I
S
T
S
More clues leading to the result RESISTS
- Is reluctant concerning relative boxing in street
- Stands up to repeat tests holding spades
- . Her Majesty upset by MI6? Time son fights back
- Takes test again about society’s struggles
- Stands up to ugly sisters
- Battles unruly sisters
- Acts in opposition, I stress, must be broken up
- Opposes breaks when touring island
- On models, section in middle sticks out
- In further tests son fights back
Buddy explains!

The straight part
We know that "Acts in opposition" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case acts in opposition = resists.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 RESISTS mean?
RESISTS noun- A protective coating or covering.
- To attempt to counter the actions or effects of.
- To withstand the actions of.
- To oppose.
- To be distasteful to.