Fungus-to-be (5)
I believe the answer is:
spore
'fungus-to-be' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for spore that I've seen before include "Asexual reproductive body" , "Ropes (anag.)" , "Fungus reproducer" , "Tiny reproductive cell of fungus or fern" , "Reproductive body produced by fungi" .)