The most common and most likely answer to this crossword clue is the 5 letter word MOREL
I've seen this crossword clue in The Times
M
O
R
E
L
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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 = morel.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 MOREL mean?
MOREL noun- Any of several edible mushrooms, especially the common morel or yellow morel.
- Any of several fungi in the genus Morchella, the upper part of which is covered with a reticulated and pitted hymenium.
- A variety of cultivated cherry, Prunus cerasus 'austera', having a dark skin