The most common and most likely answer to this crossword clue is the 9 letter word FLEDGLING
I've seen this crossword clue in The Daily Telegraph
F
L
E
D
G
L
I
N
G
Buddy explains!
The straight part
We know that "Youngster on first flight" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case youngster on first flight = fledgling.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.
Crossword Answer definitions
FLEDGLING noun- A young bird which has just developed its flight feathers (notably wings).
- An insect that has just fledged, i.e. undergone its final moult to become an adult or imago.
- An immature, naïve or inexperienced person.
- Untried or inexperienced.
- Emergent or rising.