Flag excursion around Rhode Island with colonel (9)
I believe the answer is:
Here is my best explanation:
'flag' is the definition.
(tricolour is a kind of flag)
'excursion around rhode island with colonel' is the subsidiary indication.
'excursion' becomes 'tour' (I've seen this before).
'around' means one lot of letters goes inside another.
'rhode island' becomes 'ri'.
'with' means one lot of letters go next to another.
'colonel' becomes 'col'.
'tour' going around 'ricol' is 'tricolour'.
(Other definitions for tricolour that I've seen before include "Having or involving three colours" and "French national flag".)