Heartlessly deluded by a document (4)
I believe the answer is:
deed
'a document' is the definition.
(I know that deed is a type of instrument)
'heartlessly deluded' is the wordplay.
'heartlessly' means to remove the middle letters.
'deluded' with its centre taken out is 'DEED'.
'by' acts as a link.
(Other definitions for deed that I've seen before include "Something done" , "Action - legal document" , "Intentional action" , "What's done" , "Legal document for transfer of property" .)