The most common and most likely answer to this crossword clue is the 4 plus 2 plus 2 letter word THAT IS SO
I've seen this crossword clue in The Guardian
T
H
A
T
I
S
S
O
More clues leading to the result THAT IS SO
Buddy explains!
The straight part
We know that "true" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case true = that is so.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.