Organisms studied by a mycologist (5)
I believe the answer is:
fungi
'organisms studied by a mycologist' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for fungi that I've seen before include "rusts?" , "Mushrooms, toadstools etc (5)" , "Spore producers" , "Yeast, etc" , "Kingdom of lower plants" .)