Flag shown by old seagoing vessel carrying queen (8)
I believe the answer is:
streamer
'flag' is the definition.
(I know that streamer is a type of flag)
'old seagoing vessel carrying queen' is the wordplay.
'old seagoing vessel' becomes 'steamer' (steam ship).
'carrying' means one lot of letters goes inside another.
'queen' becomes 'r' (abbreviation for regina, e.g. in ER ).
'steamer' going around 'r' is 'STREAMER'.
'shown by' is the link.
(Other definitions for streamer that I've seen before include "Long decorative ribbon - thin flag" , "Long flag, tapering perhaps" , "Paper ribbon - thin flag" , "Item of bunting" , "bold headline" .)