Queen and king under British flag (6)
I believe the answer is:
banner
'flag' is the definition.
(I know that banner is a type of flag)
'queen and king under british' is the wordplay.
'queen' becomes 'anne' (Queen Anne, 18th-century queen).
'and' says to put letters next to each other.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'under' says to put letters next to each other.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'anne' put next to 'r' is 'anner'.
'anner' after 'b' is 'BANNER'.
(Other definitions for banner that I've seen before include "Website advertisement" , "Streamer, flag" , "protester's statement?" , "Military standard" , "Kind of flag - sounds like censor" .)