The most common and most likely answer to this crossword clue is the 3 plus 3 plus 5 letter word CUT AND DRIED
I've seen this crossword clue in The Daily Telegraph
C
U
T
A
N
D
D
R
I
E
D
More clues leading to the result CUT AND DRIED
Buddy explains!
The straight part
We know that "beyond doubt" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case beyond doubt = cut and dried.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.