The most common and most likely answer to this crossword clue is the 6 plus 1 plus 5 letter word MÉNAGE À TROIS
I've seen this crossword clue in Financial Times
M
Ã
‰
N
A
G
E
Ã
€
T
R
O
I
S
More clues leading to the result MÉNAGE À TROIS
- Blokes get on with Rosa, becoming involved with it?
- Faulty gasometer in a kind of household
- Unconventional household as people get older, or as it evolves
- Lois stripped to take part in wild East German threesome
- Toulouse-Lautrec’s crowded affair?
- People mature — or it’s a disastrous sort of relationship
- Organise mate with alternative arrangement in this!
- A gnome’s irate at fault in asymmetrical arrangement
- Entanglement with Amos, Gene and Rita?
- Crowded House maybe playing a stage in Rome
Buddy explains!

The straight part
We know that "Toulouse-Lautrec’s crowded affair" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case toulouse-lautrec’s crowded affair = mÉnage À trois.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.