The most common and most likely answer to this crossword clue is the 4 plus 7 plus 4 letter word GIVE ONESELF AWAY
I've seen this crossword clue in The Guardian
G
I
V
E
O
N
E
S
E
L
F
A
W
A
Y
Buddy explains!
The straight part
We know that "Accidentally reveal/Accidentally reveal" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case accidentally reveal/accidentally reveal = give oneself away.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.