Thallophyte (6)
I believe the answer is:
fungus
'thallophyte' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for fungus that I've seen before include "Mushroom, for instance, is one" , "Mushroom, for example" , "Organism such as a toadstool" , "'Mushroom or toadstool, say (6)'" , "Plant like mushroom or toadstool" .)