So as it is the 5th clue 12a has an extra E to the parsing (5th letter of the alphabet. Therefore as the doctored clues go First half, second half, first half etc in order, as this is the 5th clue, it is the first half that is doctored
"it's for through traffic, by the way - first call to topless girl to Rome brought back reminders"
The first part is doctored!
Its for through traffic, (ring road) by the way (road) first call (ring)
Now add the E and anagram it to form an answer 3,6
RINGROADE anagram = RIO GRANDE
The second part of the clue going in the right hand grid is ordinary
MEMORANDA = Rome backwards "in" (A)manda
Hope that all explains it better!