Decided to be heartless, in fact (4)
I believe the answer is:
deed
'fact' is the definition.
(thesaurus)
'decided to be heartless' is the wordplay.
'to be heartless' suggests removing the centre.
'decided' with its middle taken out is 'DEED'.
'in' is the link.
(Other definitions for deed that I've seen before include "Feat" , "Document showing ownership" , "Legal document for transfer of property" , "documentary evidence" , "Heroic exploit" .)