The most common and most likely answer to this crossword clue is the 8 letter word INVADING
I've seen this crossword clue in The Guardian
I
N
V
A
D
I
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G
Buddy explains!

The straight part
We know that "Intruding" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case intruding = invading.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 INVADING mean?
INVADING verb- To move into.
- To enter by force in order to conquer.
- To infest or overrun.
- To attack; to infringe; to encroach on; to violate.
- To make an unwelcome or uninvited visit or appearance, usually with an intent to cause trouble or some other unpleasant situation.