Swift, heartless attack (4)
I believe the answer is:
raid
'attack' is the definition.
(raid is a kind of attack)
'swift heartless' is the wordplay.
'swift' becomes 'rapid' (I've seen this before).
'heartless' says to hollow out the word (remove centre letters).
'rapid' with its middle removed is 'RAID'.
(Other definitions for raid that I've seen before include "Smash-and-grab?" , "Invasion" , "Sudden swift attack" , "Sudden and surprise attack" , "Assault, attack" .)