As yet strange fungus (5)
I believe the answer is:
yeast
'fungus' is the definition.
(yeast is a kind of fungus)
'as yet strange' is the wordplay.
'strange' indicates an anagram.
'as'+'yet'='asyet'
'asyet' anagrammed gives 'YEAST'.
(Other definitions for yeast that I've seen before include "As yet (anag)" , "Androgynous" , "Dough-raising agent" , "Baker's ingredient" , "Fungus; food supplement" .)