Only half rate this beast (3)
I believe the answer is:
ass
'this beast' is the definition.
(I've seen this before)
'only half rate' is the wordplay.
'only half' means to take half the letters.
'rate' becomes 'assess' (I've seen this before).
'assess' cut in half is 'ASS'.
(Other definitions for ass that I've seen before include "Titania loved one" , "Dolt" , "Transport for a wise man - or a fool" , "Idiot; animal" , "Silly fellow in passionate state" .)