As yet, active in fermentation (5)
I believe the answer is:
yeast
'fermentation' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'as yet active' is the wordplay.
'active' indicates an anagram (the letters are active/moving).
'as'+'yet'='asyet'
'asyet' with letters rearranged gives 'YEAST'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for yeast that I've seen before include "Baker's raising agent" , "Androgynous" , "What makes bread rise" , "Dough-raising agent" , "Fungus able to convert sugar into alcohol" .)