Liaison to break up, leading to loneliness (9)
I believe the answer is:
isolation
'loneliness' is the definition.
(thesaurus)
'liaison to break up' is the wordplay.
'break up' indicates an anagram.
'liaison'+'to'='liaisonto'
'liaisonto' anagrammed gives 'ISOLATION'.
'leading to' acts as a link.
(Other definitions for isolation that I've seen before include "keeping apart" , "Separation, solitariness" , "Remote and lonely state" , "Detachment" , "Quarantine; solitude" .)