The most common and most likely answer to this crossword clue is the 7 letter word TREKKIE
I've seen this crossword clue in Independent
T
R
E
K
K
I
E
More clues leading to the result TREKKIE
Buddy explains!
The straight part
We know that "Sci-fi fan" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sci-fi fan = trekkie.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.