The most common and most likely answer to this crossword clue is the 7 letter word RUSTLED
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Buddy explains!

The straight part
We know that "whispered" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case whispered = rustled.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 RUSTLED mean?
RUSTLED verb- To move (something) with a soft crackling sound.
- To make or obtain in a lively, energetic way.
- To steal (cattle or other livestock).