Was said without thinking, as one left (7,3)

'was said without thinking' is the definition.
I don't know anything about this answer so I cannot judge whether this works.

'one left' is the wordplay.
I cannot quite see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
'left' could be 'l' (common abbreviation) and 'l' is found within the answer.
This explanation may well be incorrect...

'as' acts as a link.

Can you help me to learn more?