An act defended heartlessly (4)
I believe the answer is:
deed
'an act' is the definition.
(I know that act can be written as deed)
'defended heartlessly' is the wordplay.
'heartlessly' means to remove the middle letters.
'defended' with its centre taken out is 'DEED'.
(Other definitions for deed that I've seen before include "documentary evidence" , "Conscious act" , "Feat" , "Action - legal document" , "Document showing ownership" .)