As extracted from yeast, still? (3)
I believe the answer is:
yet
'still?' is the definition.
(yet can mean still or so far)
'as extracted from yeast' is the wordplay.
'extracted from' is a deletion indicator.
'yeast' with 'as' removed is 'YET'.
(Other definitions for yet that I've seen before include "Still, so far" , "Although" , "'But, however (3)'" , "in spite of that" , "Up till now" .)