Think back regarding main road south into French city (9)
I believe the answer is:
reminisce
'think back' is the definition.
(reminiscing is a kind of thinking back)
'regarding main road south into french city' is the wordplay.
'regarding' becomes 're'.
'main road' becomes 'mi'.
'south' becomes 's' ('S' can be a synonym of 'south').
'into' indicates putting letters inside.
'french city' becomes 'nice'.
's' put into 'nice' is 'nisce'.
're'+'mi'+'nisce'='REMINISCE'
(Other definitions for reminisce that I've seen before include "Indulge in recollection" , "nostalgic" , "talk about the old days" , "Remember past" , "Have fond recollections" .)