The most common and most likely answer to this crossword clue is the 9 letter word RENDERING
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More clues leading to the result RENDERING
Buddy explains!

The straight part
We know that "first layer of plaster" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case first layer of plaster = rendering.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 RENDERING mean?
RENDERING verb- (ditransitive) To cause to become.
- To interpret, give an interpretation or rendition of.
- To translate into another language.
- To pass down.
- To make over as a return.
- To give; to give back; to deliver.
- To give up; to yield; to surrender.
- To transform (a model) into a display on the screen or other media.
- To capture and turn over to another country secretly and extrajudicially.
- To convert waste animal tissue into a usable byproduct.
- To have fat drip off meat from cooking.
- To cover a wall with a layer of plaster.
- To pass; to run; said of the passage of a rope through a block, eyelet, etc.
- To yield or give way.
- To return; to pay back; to restore.
- To inflict, as a retribution; to requite.
- The act or process by which something is rendered.
- Version; translation.
- Sketch, illustration, or painting.
- The process of producing an image from an internal model, or the image thus produced.