Swift's heartless attack (4)
I believe the answer is:
raid
'attack' is the definition.
(I know that raid is a type of incursion)
'swift's heartless' is the wordplay.
'swift' becomes 'rapid' (I've seen this before).
'heartless' suggests removing the centre.
'rapid' with its centre removed is 'RAID'.
(Other definitions for raid that I've seen before include "Enter a territory and take spoils" , "Sudden and surprise attack" , "A sudden military attack and withdrawal" , "Sudden short attack" , "Stock Exchange activity" .)