The beast left one with no comeback (4)
I believe the answer is:
lion
'the beast' is the definition.
(type of animal)
'left one with no comeback' is the wordplay.
'left' becomes 'l' (common abbreviation).
'one' becomes 'i' (Roman numeral).
'with' says to put letters next to each other.
'comeback' says the letters should be written backwards.
'no' back-to-front is 'on'.
'l'+'i'+'on'='LION'
(Other definitions for lion that I've seen before include "Simba, for example" , "The -, the Witch and the Wardrobe" , "King of sorts" , "Big cat with pride" , "cowardly character in Oz" .)