One working, as yet, in wine-making (5)
I believe the answer is:
yeast
'one working as yet in wine-making' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'one working as yet' is the wordplay.
'one working' is an anagram indicator.
'as'+'yet'='asyet'
'asyet' is an anagram of 'YEAST'.
Can you help me to learn more?
(Other definitions for yeast that I've seen before include "Leavening agent in bread-making" , "Beer froth agent" , "Fungus used in fermenting" , "As yet (anag)" , "Fungus used in brewing and baking" .)