Heartless creature is worst (4)
I believe the answer is:
best
'is worst' is the definition.
(both can mean to beat someone)
'heartless creature' is the wordplay.
'heartless' says to hollow out the word (remove centre letters).
'creature' becomes 'beast' (synonyms).
'beast' with its middle removed is 'BEST'.
(Other definitions for best that I've seen before include "Beat" , "Most suitably" , "The most talented" , "Peerless" , "Optimum" .)