The most common and most likely answer to this crossword clue is the 8 letter word OBSESSED
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Buddy explains!

The straight part
We know that "fixated" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fixated = obsessed.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 OBSESSED mean?
OBSESSED verb- (passive, constructed with "with") To be preoccupied with a single topic or emotion.
- To dominate the thoughts of someone.
- (construed with over) To think or talk obsessively about.
- Intensely preoccupied with or by a given topic or emotion; driven by a specified obsession.
- Influenced or controlled by evil spirits, but less than possessed in that the spirits do not actually reside in the victim.