Girl touring island, an American state (9)
I believe the answer is:
Here is my best explanation:
'american state' is the definition.
(Louisiana is an example)
'girl touring island an' is the subsidiary indication.
'girl' becomes 'louisa' (**).
'touring' indicates putting letters inside.
'island' becomes 'i'.
'louisa' placed around 'ian' is 'louisiana'.
(Other definitions for louisiana that I've seen before include "US state, capital Baton Rouge", "State of southern US", "Southern United State on the Gulf of Mexico", "State named for French king", "IOU a snail over in the US state", "that's near Mississippi", "Southern US state", "American state, capital Baton Rouge" and "US southern state".)