The most common and most likely answer to this crossword clue is the 4 plus 7 letter word OPEN VERDICT
I've seen this crossword clue in The Guardian
O
P
E
N
V
E
R
D
I
C
T
Buddy explains!
The straight part
We know that "here reas/on for /bein/g late remains unknown" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case here reas/on for /bein/g late remains unknown = open verdict.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.