Ferret's food (4)
I believe the answer is:
grub
I believe this is a double definition.
'ferret's' is the first definition.
('grub' can be a synonym of 'ferret')
'food' is the second definition.
(grub is a kind of food)
(Other definitions for grub that I've seen before include "food, commonly" , "Insect larva or slang food (4)" , "Worm-like larva of an insect" , "Slangy food or kind of larva" , "Larva; dig shallowly" .)