Fungus as yet to be analysed (5)
I believe the answer is:
yeast
'fungus' is the definition.
(yeast is a kind of fungus)
'as yet to be analysed' is the wordplay.
'to be analysed' indicates an anagram.
'as'+'yet'='asyet'
'asyet' with letters rearranged gives 'YEAST'.
(Other definitions for yeast that I've seen before include "Fermenting agent" , "Leavening agent in bread-making" , "Organism causing fermentation" , "Fungus used to ferment sugar" , "lifting agent" .)