Search for food (4)
I believe this is a double definition.
'search' is the first definition.
(I know that grub is a more specific form of search)
'food?' is the second definition.
(grub can informally mean food)
'for' is the link.
(Other definitions for grub that I've seen before include "Larva; dig shallowly" , "Food (informal)" , "Larva; food (sl.)" , "Nosh - larva" , "Caterpillar - food" .)
'search for food' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for forage that I've seen before include "Hunt for food or provisions" , "sort of cap" , "Rummage, search for" , "Plunder; rummage" , "Wander and feed" .)