The most common and most likely answer to this crossword clue is the 10 letter word FREEBOOTER
I've seen this crossword clue in The Daily Telegraph
F
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B
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More clues leading to the result FREEBOOTER
Buddy explains!

The straight part
We know that "for pirate" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case for pirate = freebooter.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 FREEBOOTER mean?
FREEBOOTER noun- An adventurer who pillages, plunders or wages ad-hoc war on other nations.
- One who rehosts online media without authorization; one who freeboots.