Crowd perceived to be heartless (4)
I believe the answer is:
herd
'crowd' is the definition.
(I know that herd is a more specific form of crowd)
'perceived to be heartless' is the wordplay.
'perceived' becomes 'heard' (hearing is a kind of perceiving).
'to be heartless' says to hollow out the word (remove centre letters).
'heard' with its middle removed is 'HERD'.
(Other definitions for herd that I've seen before include "Bunch of cattle" , "Collection of cattle" , "Group of people behaving like sheep?" , "Flock" , "People in the mass" .)