The most common and most likely answer to this crossword clue is the 12 letter word INFILTRATION
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Buddy explains!

The straight part
We know that "Secret entry" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case secret entry = infiltration.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 INFILTRATION mean?
INFILTRATION noun- The act or process of infiltrating, as of water into a porous substance, or of a fluid into the cells of an organ or part of the body.
- The substance which has entered the pores or cavities of a body.
- The act of secretly entering a physical location and/or organization.