The most common and most likely answer to this crossword clue is the 7 plus 1 letter word LACONIC A
I've seen this crossword clue in Independent
L
A
C
O
N
I
C
A
Buddy explains!
The straight part
We know that "Sententiously brief" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sententiously brief = laconic a.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.
What does LACONIC A mean?
LACONIC A noun- A hot dry sweating room, next to the caldarium in the Roman baths.