The most common and most likely answer to this crossword clue is the 13 letter word CHESTERFIELDS
I've seen this crossword clue in The Times
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Buddy explains!

The straight part
We know that "furniture to sit in" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case furniture to sit in = chesterfields.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 CHESTERFIELDS mean?
CHESTERFIELDS noun- A couch, sofa, or love seat with padded arms and back of the same height, often curved outward at the top.
- Any couch or sofa.