Many beasts are heard to be heartless (4)
I believe the answer is:
herd
'many beasts are' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'heard to be heartless' is the wordplay.
'to be heartless' suggests removing the centre.
'heard' with its middle taken out is 'HERD'.
Can you help me to learn more?
(Other definitions for herd that I've seen before include "Stock of cattle" , "Group of cows, elephants etc" , "Large group of animals" , "Assemblage" , "Collection of cattle" .)