Flag of British monarch beginning to reign (6)
I believe the answer is:
banner
'flag' is the definition.
(I know that banner is a type of flag)
'british monarch beginning to reign' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'monarch' becomes 'anne' (Queen Anne, 18th-century queen).
'beginning to' suggests taking the first letters.
The first letter of 'reign' is 'r'.
'b'+'anne'+'r'='BANNER'
'of' acts as a link.
(Other definitions for banner that I've seen before include "Standard carried at demo etc" , "The flag for one who prohibits" , "Long strip with a slogan" , "Flag; headline" , "Queen brought in British flag" .)