An act heartlessly denied (4)
I believe the answer is:
deed
'an act' is the definition.
(I know that act can be written as deed)
'heartlessly denied' is the wordplay.
'heartlessly' says to hollow out the word (remove centre letters).
'denied' with its middle removed is 'DEED'.
(Other definitions for deed that I've seen before include "Act - exploit - legal document" , "Something done" , "Document showing ownership" , "Action - legal document" , "Legal document for transfer of property" .)