The most common and most likely answer to this crossword clue is the 6 letter word DHARNA
D
H
A
R
N
A
Buddy explains!

The straight part
We know that "Demand for justice" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case demand for justice = dharna.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.
What does DHARNA mean?
DHARNA noun- A nonviolent sit-in protest.
- (specifically) A fast undertaken at the door of an offender, especially a debtor.