Grub is something very hot, reportedly (5)
I believe the answer is:
larva
'grub' is the definition.
(I know that grub can be written as larva)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for larva that I've seen before include "Insect just out of egg" , "Juvenile form of insect" , "E.g. caterpillar form" , "Caterpillar perhaps" , "Immature stage of an insect" .)