The most common and most likely answer to this crossword clue is the 14 letter word SLEEP-LEARNING
I've seen this crossword clue in Financial Times
S
L
E
E
P
-
L
E
A
R
N
I
N
G
Buddy explains!
The straight part
We know that "study overnight" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case study overnight = sleep-learning.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.