Prompt heartless attack (4)
I believe the answer is:
raid
'attack' is the definition.
(I know that raid is a more specific form of the action attack)
'prompt heartless' is the wordplay.
'prompt' becomes 'rapid' (I've seen this before).
'heartless' says to hollow out the word (remove centre letters).
'rapid' with its middle taken out is 'RAID'.
(Other definitions for raid that I've seen before include "Assault, attack" , "Sudden and surprise attack" , "Invade" , "Rifle" , "Inroad" .)