A ribbon round the royal vessel (8)
I believe the answer is:
streamer
'a ribbon' is the definition.
(I've seen this before)
'round the royal vessel' is the wordplay.
'round' indicates putting letters inside.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'royal' becomes 'r' (abbreviation).
'vessel' becomes 'seamer' (I can't justify this - if you can you should give a lot more credence to this answer).
't'+'r'='tr'
'tr' placed into 'seamer' is 'STREAMER'.
Can you help me to learn more?
(Other definitions for streamer that I've seen before include "Long narrow ribbon for er, master" , "Long flag -- bad cold" , "Ribbon flowing in the wind" , "bold headline" , "Banner, flag; fishing fly" .)