Agree to embrace old lover, it leads to complication (10)
I believe the answer is:
complexity
'complication' is the definition.
(thesaurus)
'agree to embrace old lover it' is the wordplay.
'agree' becomes 'comply' (complying is a kind of agreeing).
'to embrace' is an insertion indicator.
'old lover' becomes 'ex' (an ex-lover).
'ex'+'it'='exit'
'comply' placed around 'exit' is 'COMPLEXITY'.
'leads to' is the link.
(Other definitions for complexity that I've seen before include "Intricacy" , "Involvement" , "intricate quality" , "elaborate" , "It's far from straightforward" .)