The most common and most likely answer to this crossword clue is the 8 letter word TRAPPIST
I've seen this crossword clue in The Guardian
T
R
A
P
P
I
S
T
More clues leading to the result TRAPPIST
- Returning part quietly, is ahead of time — he says little
- Brother keeping mum
- Who doesn’t speak of the Sound of Music family supporter?
- One promising to keep mum permanently in gin, drunk by the sound of it?
- Brother’s reportedly drunk on gin
- Member silenced following point of order?
- One who vows to be silent
- Man who silently obeys order
- Gin on piano – it’s out for he who keeps mum
- Retiring role mine, keeping shtoom for a start?
Buddy explains!
The straight part
We know that "One promising to keep mum permanently" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one promising to keep mum permanently = trappist.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.