Forms of fungi (6)
I believe the answer is:
moulds
This clue is a double definition.
'forms' is the first definition.
(I know this)
'fungi' is the second definition.
(mould is a kind of fungus)
'of' acts as a link.
(Other definitions for moulds that I've seen before include "Frames or cavities for wax, etc." , "Containers into which liquid is poured to create shapes" , "Hollow containers used to give shapes to hot liquid material" , "Hollow shapes for molten liquid material" , "Frames for clay, wax, etc" .)