Grub right inside hot rock (5)
I believe the answer is:
larva
'grub' is the definition.
(I've seen this before)
'right inside hot rock' is the wordplay.
'right' becomes 'r' (common abbreviation).
'inside' indicates putting letters inside.
'hot rock' becomes 'lava' (I've seen this in another clue).
'r' placed inside 'lava' is 'LARVA'.
(Other definitions for larva that I've seen before include "Insect in the first post-egg stage of its life" , "Manage" , "Insect's feeding stage" , "Caterpillar perhaps" , "early form" .)