Bank that's really heartless (4)
I believe the answer is:
rely
'bank' is the definition.
(thesaurus)
'really heartless' is the wordplay.
'heartless' means to remove the middle letters.
'really' with its centre taken out is 'RELY'.
'that's' acts as a link.
(Other definitions for rely that I've seen before include "City with cathedral" , "All the letters" , "Depend or lean on" , "Depend on with confidence" , "Old rally" .)