Threesome in extremely adventurous liaison, primarily relating in complicated way (7,8)
I believe the answer is:
eternal triangle
'threesome in extremely adventurous liaison' is the definition.
'eternal triangle' can be an answer for 'liaison' (thesaurus). I am not sure about the 'threesome in extremely adventurous' bit.
'primarily relating in complicated way' is the wordplay.
I cannot really understand how this works, but
an anagram of 'relating' is 'triangle' which is located in the answer.
'way' could be 'n' (Compass direction - North) and 'n' is found in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for eternal triangle that I've seen before include "Love relationship between three people" , "emotional relationship" , "Complex relationship" , "In which three lovers" , "sexual relationship" .)