Caveman discovery
I believe the answer is:
fire
'caveman discovery' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for fire that I've seen before include "Set light to; shoot" , "Ready, aim . . . ." , "Flame - dismiss - shoot" , "Dismiss; bake in a kiln" , "Blaze - sack" .)