Heartlessly declaims traitors (4)
I believe the answer is:
rats
'traitors' is the definition.
'heartlessly declaims' is the wordplay.
'heartlessly' says to hollow out the word (remove centre letters).
'declaims' becomes 'rants' (to declaim is to rant or speak passionately).
'rants' with its centre taken out is 'RATS'.
(Other definitions for rats that I've seen before include "Annoyed expression" , "Mild oath - sewer residents" , "(Hunts) rodents - expression of irritation" , "deserters" , "Rodent pests" .)