He's monstrous and so backward (4)
I believe the answer is:
ogre
'he's monstrous' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'so backward' is the wordplay.
'so' becomes 'ergo' (synonyms).
'backward' is a reversal indicator.
'ergo' written backwards gives 'OGRE'.
'and' is the link.
Can you help me to learn more?
(Other definitions for ogre that I've seen before include "character in 'Puss in Boots'" , "Giant: monster" , "Bugbear" , "Human-eating giant" , "The giant is some goer" .)