The most common and most likely answer to this crossword clue is the 11 letter word LIBERTARIAN
I've seen this crossword clue in The Times
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More clues leading to the result LIBERTARIAN
- African kidnapping sailor is opposed to authority
- Turning irritable with an advocate of free will
- African imprisoning sailor who’s opposed to oppressive government?
- Sailor introduced to African as believer in free will
- African haven for sailor — one rejecting all limits
- Tuneful Ibert aria now involves a freedom lover
Buddy explains!

The straight part
We know that "believer in free will" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case believer in free will = libertarian.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 LIBERTARIAN mean?
LIBERTARIAN noun- One who advocates liberty, either generally or in relation to a specific issue.
- A believer in a political doctrine that emphasizes individual liberty and a lack of governmental regulation, intervention, and oversight both in matters of the economy (‘free market’) and in personal behavior where no one’s rights are being violated or threatened; also, a ‘classical liberal’, akin to an ‘anarcho-capitalist’.
- A left-libertarian, an antiauthoritarian believer in both individual freedom and social justice (social equality and mutual aid), such as a social anarchist.
- A believer in the freedom of thinking beings to choose their own destiny, i.e. a believer in free will as opposed to those who believe the future is predetermined.
- Having the beliefs of libertarians; having a relative tendency towards liberty.
- Relating to liberty, or to the doctrine of free will, as opposed to the doctrine of necessity.