Of old barbarians, a French one at Southend? (3)
I believe the answer is:
hun
'of old barbarians' is the definition.
The definition suggests a singular noun which matches the answer.
'a french one at southend?' is the wordplay.
'a french' becomes 'un' ('a' in French).
'one at' says to put letters next to each other (I've seen this in other clues).
'southend?' becomes 'h' (I can't justify this - if you can you should believe this answer much more).
'un' put after 'h' is 'HUN'.
Can you help me to learn more?
(Other definitions for hun that I've seen before include "old invader" , "Eg, Attila" , "One of Attila's people" , "Warlike invader" , "Attila the ... invaded Europe in 5th century" .)