The most common and most likely answer to this crossword clue is the 9 letter word CRYPTOGAM
I've seen this crossword clue in The Times
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The straight part
We know that "Eg fungus" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case eg fungus = cryptogam.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.
Crossword Answer definitions
CRYPTOGAM noun- Any plant that reproduces using spores (rather than seeds), formerly placed in the taxonomic group Cryptogamae, which included ferns, mosses, algae, fungi, lichens and liverworts.