Bank very heartless (4)
I believe the answer is:
rely
'bank' is the definition.
(to bank on something is to rely on it)
'very heartless' is the wordplay.
'very' becomes 'really' (I've seen this before).
'heartless' says to hollow out the word (remove centre letters).
'really' with its middle taken out is 'RELY'.
(Other definitions for rely that I've seen before include "Run and see" , "Depend (upon)" , "Be sure" , "Old rally" , "Trust, depend" .)