The most common and most likely answer to this crossword clue is the 11 letter word COUNTERSIGN
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More clues leading to the result COUNTERSIGN
Buddy explains!

The straight part
We know that "Initial" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case initial = countersign.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 COUNTERSIGN mean?
COUNTERSIGN noun- A second signature added to a document to affirm the validity of the signature of the first person.
- The response to a sign or signal.
- To sign on the opposite side of (a document).
- (by extension) To add a second signature to a document, affirming the validity of the signature of another person.