The most common and most likely answer to this crossword clue is the 2 plus 4 letter word DE FIDE
D
E
F
I
D
E
Buddy explains!
The straight part
We know that "That’s got to be believed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that’s got to be believed = de fide.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.