The most common and most likely answer to this crossword clue is the 9 letter word SERPENTRY
S
E
R
P
E
N
T
R
Y
Buddy explains!
The straight part
We know that "Occupants of reptile house" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case occupants of reptile house = serpentry.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.