The most common and most likely answer to this crossword clue is the 9 letter word ANTI-HERO
I've seen this crossword clue in The Guardian
A
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More clues leading to the result ANTI-HERO
Buddy explains!

The straight part
We know that "unconventional leading man" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unconventional leading man = anti-hero.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.
What does ANTI-HERO mean?
ANTI-HERO noun- A protagonist who proceeds in an unheroic manner, such as by criminal means, via cowardly actions, or for mercenary goals.