From Oxford Dictionaries:
A person filled with excessive and single-minded zeal, especially for an extreme religious or political cause:
In everyday speech, ‘fanatic’ can be replaced with ‘crazy about’ or ‘fan’ or another term to describe a person who is outrageously passionate about something. While this is usually acceptable in casual conversation, it is important to note that ‘fanatic’ is a more specific noun and should be used as such in writing.
In a narrative piece:
Sarah dreaded going to her political studies tutorial every Tuesday; the Australian Greens fanatic would inevitably embark on a rant about political inaction on global warming.
In a text essay:
The discomfort that the Australian Greens fanatic caused in Sarah triggers her hallucinations and marks the beginning of her descent into madness.
Note that in both examples, ‘fanatic’ is used as a noun and is further described by the phrase ‘Australian Greens’, which acts as a descriptor to add to the reader’s understanding of what Sarah’s classmate is passionate about.
I hope that was helpful!