Where long-term partners are going, yet sad to break up (6)
I believe the answer is:
steady
'where long-term partners are going' is the definition.
('going steady' means in a long-term relationship)
'yet sad to break up' is the wordplay.
'to break up' indicates anagramming the letters.
'yet'+'sad'='yetsad'
'yetsad' anagrammed gives 'STEADY'.
(Other definitions for steady that I've seen before include "On an even keel" , "Well balanced and stable" , "Changeless, constant" , "Firmly supported or balanced" , "Stable - boyfriend or girlfriend" .)