Immature creature runs through hot rocks (5)
I believe the answer is:
larva
'immature creature' is the definition.
'runs through hot rocks' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'through' means one lot of letters goes inside another (inserted letters go through the word).
'hot rocks' becomes 'lava' (I've seen this in another clue).
'r' put within 'lava' is 'LARVA'.
(Other definitions for larva that I've seen before include "Animal in immature state" , "a developmental stage" , "shows form of immaturity" , "Insect's feeding stage" , "Immature form of caterpillar" .)