The most common and most likely answer to this crossword clue is the 13 letter word SCHOOLGIRLISH
I've seen this crossword clue in The Times
S
C
H
O
O
L
G
I
R
L
I
S
H
Buddy explains!
The straight part
We know that "reminiscent of a young lass" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case reminiscent of a young lass = schoolgirlish.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.