The most common and most likely answer to this crossword clue is the 5 letter word FICUS
I've seen this crossword clue in The Times
F
I
C
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S
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The straight part
We know that "house plant?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case house plant? = ficus.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 FICUS mean?
FICUS noun- A plant belonging to the genus Ficus, including the rubber plant.