Fungus is still, when incorporated (5)
I believe the answer is:
yeast
'fungus' is the definition.
(I know that yeast is a type of fungus)
'still when incorporated' is the wordplay.
I cannot quite see how this works, but
'when' could be 'as' (eg 'when I walked' means 'as I walked') and 'as' is found in the answer.
'still' could be 'yet' (yet can mean still or so far) and 'yet' is found in the remaining letters.
This accounts for all the letters.
This explanation may well be incorrect...
'is' acts as a link.
Can you help me to learn more?
(Other definitions for yeast that I've seen before include "Fungus used in fermenting" , "Beer froth agent" , "As yet (anag)" , "Agent for making bread" , "Baker's ingredient" .)