The most common and most likely answer to this crossword clue is the 5 letter word GROUT
I've seen this crossword clue in Independent
G
R
O
U
T
More clues leading to the result GROUT
Buddy explains!

The straight part
We know that "plaster" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case plaster = grout.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 GROUT mean?
GROUT noun- A thin mortar used to fill the gaps between tiles and cavities in masonry.
- Coarse meal; groats.
- (typically used in the plural) Dregs, sediment.
- A kind of beer or ale.
- To insert mortar between tiles.