The most common and most likely answer to this crossword clue is the 7 plus 8 letter word ETERNAL TRIANGLE
I've seen this crossword clue in Independent
E
T
E
R
N
A
L
T
R
I
A
N
G
L
E
More clues leading to the result ETERNAL TRIANGLE
- Threesome in extremely adventurous liaison, primarily relating in complicated way
- Constant problem with relating in ménage à trois
- Does it always figure in marriage break-ups?
- Unfolded letter, learning about a tense relationship
- Perpetual figure that causes matrimonial disputes
- Trio finding harmony difficult are lent unusually simple instrument
- Ageless integral form of relationship
- In which three lovers linger and alternate excitedly?
- Threesome without end relating in complex way?
Buddy explains!
The straight part
We know that "ménage à trois" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ménage à trois = eternal triangle.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.