The most common and most likely answer to this crossword clue is the 8 letter word MILITATE
I've seen this crossword clue in Financial Times
M
I
L
I
T
A
T
E
Buddy explains!
The straight part
We know that "to fight for a cause" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to fight for a cause = militate.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 MILITATE mean?
MILITATE verb- To give force or effect toward; to influence.
- To fight.