Toadstool or mushroom, eg (6)
I believe the answer is:
fungus
This clue is a double definition.
'toadstool' is the first definition.
(I've seen this before)
'mushroom eg' is the second definition.
(I know that mushroom is a type of basidiomycete)
(Other definitions for fungus that I've seen before include "A mushroom, toadstool or mould" , "Mould or similar growth" , "'Lichen, say (6)'" , "Mushroom, for instance, is one" , "One as yeast, mushroom" .)