The most common and most likely answer to this crossword clue is the 3 plus 5 plus 4 letter word YOU DON’T SAY!
I've seen this crossword clue in The Guardian
Y
O
U
D
O
N
â
€
™
T
S
A
Y
!
Buddy explains!
The straight part
We know that "that is so obvious" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that is so obvious = you don’t say!.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.