Cryptic crossword clue
Girl's description of operational ship impervious to attack 12

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

I've seen this crossword clue in The Times
U
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A
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Buddy explains!

The straight part
We know that "impervious to attack" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case impervious to attack = unassailable.
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 UNASSAILABLE mean?

UNASSAILABLE noun
  1. Something, such as a belief, that cannot be assailed.
UNASSAILABLE adjective
  1. Secure against attack; impregnable.
  2. (by extension) Undeniable, incontestable or incontrovertible.

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