British princess, first to raise flag (6)
I believe the answer is:
banner
'flag' is the definition.
(I know that banner is a type of flag)
'british princess first to raise' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'princess' becomes 'Anne' (Princess Anne, daughter of Queen Elizabeth).
'first to' indicates taking the first letters.
The first letter of 'raise' is 'r'.
'b'+'anne'+'r'='BANNER'
(Other definitions for banner that I've seen before include "Standard carried at demo etc" , "Large fabric sign" , "Flag bearing a device" , "Long strip with a slogan" , "Tree product; series of games" .)