Cryptic crossword clue
Merchant once foolishly making intrusive entry 12

The most common and most likely answer to this crossword clue is the 12 letter word ENCROACHMENT

I've seen this crossword clue in Financial Times
E
N
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A
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T

Buddy explains!

The straight part
We know that "intrusive entry" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case intrusive entry = encroachment.
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 ENCROACHMENT mean?

ENCROACHMENT noun
  1. An entry into a place or area that was previously uncommon; an advance beyond former borders; intrusion; incursion.
  2. An intrusion upon another's possessions or rights; infringement.
  3. That which is gained by such unlawful intrusion.
  4. An unlawful diminution of the possessions of another.

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