The most common and most likely answer to this crossword clue is the 8 plus 7 letter word SPEAKING TRUMPET
I've seen this crossword clue in The Guardian
S
P
E
A
K
I
N
G
T
R
U
M
P
E
T
More clues leading to the result SPEAKING TRUMPET
Buddy explains!
The straight part
We know that "means to be heard" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case means to be heard = speaking trumpet.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.