Biscuit is extremely vile after regularly adding yeast to it (9)
I believe the answer is:
digestive
'biscuit' is the definition.
(I've seen this before)
'is extremely vile after regularly adding yeast to it' is the wordplay.
'is extremely' means to remove the middle letters (only the extremes of the word are used).
'after' is a charade indicator (letters next to each other).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'vile' with its middle removed is 've'.
'adding'+'yeast'+'to'+'it'='addingyeasttoit'
The alternating letters of 'addingyeasttoit' are 'digesti'.
've' after 'digesti' is 'DIGESTIVE'.
(Another definition for digestive that I've seen is " Type of biscuit".)