Though backward, he sounds more advanced (5)
I believe the answer is:
moron
'though backward he sounds more advanced' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he sounds more advanced' is the wordplay.
I cannot really understand how this works, but
'he' could be 'ron' (short for Ronald) and 'ron' is present in the answer.
'more' could be 'mo' (I've seen this in other clues) and 'mo' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for moron that I've seen before include "Person of subnormal intelligence" , "Cretin" , "Foolish or stupid person" , "Somewhat feeble-minded person" , "Idiot" .)