Not married -- ambition I abandoned from the start (2,6)
I believe the answer is:
ab initio
'from the start' is the definition.
(I've seen this before)
'not married ambition i abandoned' is the wordplay.
'not' suggests deleting specific letters.
'married' becomes 'm'.
'abandoned' indicates an anagram.
'ambition'+'i'='ambitioni'
'ambitioni' with 'm' taken out is 'abitioni'.
'abitioni' is an anagram of 'AB INITIO'.
(Other definitions for ab initio that I've seen before include "From the start - in Latin" , "from scratch" , "From the beginning in Latin" , "from the outset" , "Undercoat for metal/wood" .)