The most common and most likely answer to this crossword clue is the 6 plus 8 letter word SILENT MAJORITY
I've seen this crossword clue in The Times
S
I
L
E
N
T
M
A
J
O
R
I
T
Y
Buddy explains!
The straight part
We know that "Unheard of numbers" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unheard of numbers = silent majority.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.