The most common and most likely answer to this crossword clue is the 4 plus 10 letter word SALT SATYAGRAHA
S
A
L
T
S
A
T
Y
A
G
R
A
H
A
Buddy explains!
The straight part
We know that "Protest against unreasonable tax" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case protest against unreasonable tax = salt satyagraha.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.