The most common and most likely answer to this crossword clue is the 6 letter word DUMBER
I've seen this crossword clue in Independent
D
U
M
B
E
R
Buddy explains!

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 DUMBER mean?
DUMBER adjective- Unable to speak; lacking power of speech (kept in "deaf, dumb, and blind").
- Silent; unaccompanied by words.
- (especially of a person) Extremely stupid.
- Pointless, foolish, lacking intellectual content or value.
- Lacking brightness or clearness, as a colour.