The most common and most likely answer to this crossword clue is the 8 plus 4 letter word TRAPPIST MONK
I've seen this crossword clue in The Guardian
T
R
A
P
P
I
S
T
M
O
N
K
Buddy explains!
The straight part
We know that "Brother who keeps mum" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case brother who keeps mum = trappist monk.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.