Go red after short excursion and flag (9)
I believe the answer is:
tricolour
'flag' is the definition.
(tricolour is a kind of flag)
'go red after short excursion' is the wordplay.
'go red' becomes 'colour' (I've seen this before).
'after' is a charade indicator (letters next to each other).
'short' means to remove the last letter.
'excursion' becomes 'trip' (I've seen this before).
'trip' with its last letter removed is 'tri'.
'colour' after 'tri' is 'TRICOLOUR'.
'and' is the link.
(Other definitions for tricolour that I've seen before include "French national flag" , "Having or involving three colours" , "Standard of French perhaps" .)