Fungus; develop rapidly (8)
I believe the answer is:
mushroom
This clue is a double definition.
'fungus' is the first definition.
(mushroom is a kind of fungus)
'develop rapidly' is the second definition.
(I've seen this before)
(Other definitions for mushroom that I've seen before include "Stroganoff ingredient" , "Snowball; fungus" , "pread quickly" , "Increase with remarkable rapidity" , "Grow very fast" .)