The most common and most likely answer to this crossword clue is the 6 letter word RESIST
I've seen this crossword clue in Financial Times
R
E
S
I
S
T
More clues leading to the result RESIST
- Support outside is making stand firm
- Fight is dividing the others
- Withstand naughty sister
- Fight is in the balance
- Hold out against dire sister to some extent
- Stand up against support one's brought in
- Oppose others around island
- One’s bound by others to put up a fight
- Explores Istanbul’s stonewall
- Fight southpaw primarily in further test
Buddy explains!

The straight part
We know that "stonewall" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case stonewall = resist.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 RESIST mean?
RESIST 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.