The most common and most likely answer to this crossword clue is the 10 plus 6 letter word DISCLOSING TABLET
I've seen this crossword clue in The Times
D
I
S
C
L
O
S
I
N
G
T
A
B
L
E
T
Buddy explains!
The straight part
We know that "One revealing plaque" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one revealing plaque = disclosing tablet.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.