The most common and most likely answer to this crossword clue is the 6 letter word HACKER
I've seen this crossword clue in Financial Times
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Buddy explains!
The straight part
We know that "Intruder" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case intruder = hacker.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 HACKER mean?
HACKER noun- One who is expert at programming and solving problems with a computer.
- One who uses a computer to gain unauthorized access to data, or to carry out malicious attacks.
- A computer security professional.
- Something that hacks; a tool or device for hacking.
- A fork-shaped tool used to harvest root vegetables.
- Someone who hacks.
- One who is inexperienced or unskilled at a particular activity, especially a sport such as golf or tennis.
- One who operates a taxicab