The most common and most likely answer to this crossword clue is the 12 letter word INCRUSTATION
I've seen this crossword clue in The Daily Telegraph
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Buddy explains!
The straight part
We know that "Hard coat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case hard coat = incrustation.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 INCRUSTATION mean?
INCRUSTATION noun- The act of incrusting, or the state of being incrusted.
- A crust or hard coating of anything upon or within a body, as a deposit of lime, sediment, etc., from water on the inner surface of a steam boiler.
- A covering or inlaying of marble, mosaic, etc., attached to the masonry by cramp irons or cement.
- Anything inlaid or embedded.