The most common and most likely answer to this crossword clue is the 11 letter word CHANTERELLE
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More clues leading to the result CHANTERELLE
Buddy explains!

The straight part
We know that "mushroom" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mushroom = chanterelle.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 CHANTERELLE mean?
CHANTERELLE noun- A widely distributed edible mushroom, Cantharellus cibarius, being yellow and trumpet-shaped; or any similar mushroom of the genera Cantharellus, Polyozellus or Gomphus, not all of which are edible.
- The highest string of the violin or similar instrument.