Apple from which grub might emerge? (6)
I believe the answer is:
cooker
I believe this clue is a double definition.
'apple' is the first definition.
(cooking apple)
'from which grub might emerge?' is the second definition.
(where food or grub might be cooked)
(Other definitions for cooker that I've seen before include "Oven" , "Kind of apple" , "Item of kitchen equipment" , "Type of apple; stove" , "Kitchen appliance" .)