One considered monstrous is therefore sent back (4)
I believe the answer is:
ogre
'one considered monstrous' is the definition.
I can't judge whether this definition defines the answer.
'therefore sent back' is the wordplay.
'therefore' becomes 'ergo' (I've seen this before).
'sent back' says the letters should be written in reverse.
'ergo' back-to-front is 'OGRE'.
'is' is the link.
Can you help me to learn more?
(Other definitions for ogre that I've seen before include "Ergo (anag.)" , "One to gobble flesh" , "Cruel giant of folklore" , "Jack's giant perhaps" , "stern one who inspires fear" .)