The most common and most likely answer to this crossword clue is the 10 letter word CAMPAIGNER
I've seen this crossword clue in The Guardian
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More clues leading to the result CAMPAIGNER
Buddy explains!

The straight part
We know that "crusader" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case crusader = campaigner.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 CAMPAIGNER mean?
CAMPAIGNER noun- A person who has served in a military campaign.
- (by extension) A military veteran.
- A person who campaigns for a person running for political office or works, or supports, in an organised and active way towards a goal .
- Someone with experience in a certain field.
- An activist.