The most common and most likely answer to this crossword clue is the 12 letter word CHANTERELLES
I've seen this crossword clue in The Daily Telegraph
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Buddy explains!
The straight part
We know that "Mushrooms" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mushrooms = chanterelles.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 CHANTERELLES mean?
CHANTERELLES 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.