The most common and most likely answer to this crossword clue is the 9 letter word CLUBHOUSE
I've seen this crossword clue in The Guardian
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More clues leading to the result CLUBHOUSE
Buddy explains!

The straight part
We know that "follower of 18?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case follower of 18? = clubhouse.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 CLUBHOUSE mean?
CLUBHOUSE noun- Any building used by a club for meetings or social activities.
- A locker room and possibly associated rooms used by an athletic team.
- A building at a golf course that houses various activities associated with golf.