Attack! Throw a stone (5)
I believe the answer is:
onset
'attack' is the definition.
(archaic term for a military attack)
'throw a stone' is the wordplay.
'throw' indicates anagramming the letters.
'stone' anagrammed gives 'ONSET'.
(Other definitions for onset that I've seen before include "Beginning or early stages" , "Stone (anag)" , "Assault; beginning" , "First signs" , "Start, beginning" .)