The most common and most likely answer to this crossword clue is the 1 plus 7 letter word A STUDENT
I've seen this crossword clue in Independent
A
S
T
U
D
E
N
T
Buddy explains!
The straight part
We know that "like Trump when younger (allegedly)" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case like trump when younger (allegedly) = a student.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.