The most common and most likely answer to this crossword clue is the 6 plus 9 letter word BUTTON MUSHROOMS
I've seen this crossword clue in The Guardian
B
U
T
T
O
N
M
U
S
H
R
O
O
M
S
More clues leading to the result BUTTON MUSHROOMS
Buddy explains!
The straight part
We know that "fungal growths" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fungal growths = button mushrooms.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.