The most common and most likely answer to this crossword clue is the 5 plus 8 letter word CLASS STRUGGLE
I've seen this crossword clue in Financial Times
C
L
A
S
S
S
T
R
U
G
G
L
E
More clues leading to the result CLASS STRUGGLE
Buddy explains!
The straight part
We know that "idea of marks in report" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case idea of marks in report = class struggle.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.