Returning thanks with poor Latin, endless hordes he led against Rome (6)
I believe the answer is:
attila
'hordes he led against rome' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'returning thanks with poor latin endless' is the wordplay.
'returning' shows that the letters should be reversed in order.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'with' is a charade indicator (letters next to each other).
'poor' indicates an anagram.
'endless' means to remove the last letter.
'ta' backwards is 'at'.
'latin' with its last letter taken off is 'lati'.
'lati' anagrammed gives 'tila'.
'at'+'tila'='ATTILA'
Can you help me to learn more?
(Other definitions for attila that I've seen before include "Renowned Hun" , "Hunnish ruler" , "King of the Huns, invader of the Roman Empire" , "Warrior" , "A very great Hun of old" .)