The most common and most likely answer to this crossword clue is the 6 letter word YUPPIE
I've seen this crossword clue in The Times
Y
U
P
P
I
E
More clues leading to the result YUPPIE
Buddy explains!

The straight part
We know that "ambitious young man" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ambitious young man = yuppie.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 YUPPIE mean?
YUPPIE noun- A young upwardly mobile urban professional person with an affluent lifestyle.