The most common and most likely answer to this crossword clue is the 7 letter word UNBOSOM
I've seen this crossword clue in The Guardian
U
N
B
O
S
O
M
Buddy explains!
The straight part
We know that "come clean" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case come clean = unbosom.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 UNBOSOM mean?
UNBOSOM verb- To tell someone about (one's troubles), and thus obtain relief.
- To free (oneself) of the burden of one's troubles by telling of them.
- To confess a misdeed.