The most common and most likely answer to this crossword clue is the 6 letter word ATTILA
A
T
T
I
L
A
More clues leading to the result ATTILA
- Old King's attic lad left incomplete
- Idiotic tit entering in the manner of fearsome leader
- Brutal invader one who cultivates land some say
- He attacked the Romans but turned tail at first
- Leader of tribe taking the helm, we hear
- Barbarian army raised and landed to the north
- Conqueror turned to race, inter alia
- Invader has trouble crossing dry area
- Old king’s a dry one, facing the Italian
- Barbarian one fired army up
Buddy explains!

The straight part
We know that "fearsome leader (…the Hun)" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fearsome leader (…the hun) = attila.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.