The most common and most likely answer to this crossword clue is the 4 plus 7 letter word HIGH COMMAND
I've seen this crossword clue in Financial Times
H
I
G
H
C
O
M
M
A
N
D
Buddy explains!
The straight part
We know that "Top brass" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case top brass = high command.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.