Heartlessly quick strike (4)
I believe the answer is:
raid
'strike' is the definition.
'heartlessly quick' is the wordplay.
'heartlessly' means to remove the middle letters.
'quick' becomes 'rapid' (synonyms).
'rapid' with its middle taken out is 'RAID'.
(Other definitions for raid that I've seen before include "Maraud" , "military operation" , "Assault, attack" , "Surprise police visit" , "Finch and Bronson starred in . . . . On Entebbe" .)