Fungus spread rapidly (8)
I believe the answer is:
mushroom
This clue could be a double definition.
'fungus' is the first definition.
(mushroom is a kind of fungus)
'spread rapidly' is the second definition.
'mushroom' can be an answer for 'spread' (thesaurus). I am unsure of the 'rapidly' bit.
Can you help me to learn more?
(Other definitions for mushroom that I've seen before include "grow quickly" , "Grow very fast" , "parasol, for instance" , "pread quickly" , "Snowball; fungus" .)