Common food of the insect larva (4)
I believe the answer is:
grub
This clue is a double definition.
'common food' is the first definition.
(I've seen this before)
'the insect larva' is the second definition.
(grub is a type of insect larva)
'of' acts as a link.
(Other definitions for grub that I've seen before include "Caterpillar - food" , "Table needs" , "Larva; food (sl.)" , "Slangy food or kind of larva" , "Beetle larva" .)