In which three lovers linger and alternate excitedly? (7,8)
I believe the answer is:
eternal triangle
'in which three lovers' is the definition.
(eternal triangle can mean a love triangle)
'linger and alternate excitedly?' is the wordplay.
'and' says to put letters next to each other.
'excitedly?' indicates an anagram (I've seen 'excitedly' mean this).
'linger' after 'alternate' is 'alternatelinger'.
'alternatelinger' anagrammed gives 'ETERNAL TRIANGLE'.
(Other definitions for eternal triangle that I've seen before include "emotional relationship" , "In which three lovers" , "Complex relationship" , "sexual relationship" , "Love relationship between three people" .)