Banner welcoming king aboard ship (8)
I believe the answer is:
streamer
'banner' is the definition.
(both can mean a flag)
'welcoming king aboard ship' is the wordplay.
'welcoming king' becomes 'r' (abbreviation for rex, king in Latin. I am not sure about the 'welcoming' bit.).
'aboard' indicates putting letters inside.
'ship' becomes 'steamer' (a steam ship).
'r' put into 'steamer' is 'STREAMER'.
Can you help me to learn more?
(Other definitions for streamer that I've seen before include "terrible cold?" , "Long narrow ribbon for er, master" , "It blows in the wind" , "Ribbon for throwing" , "Narrow decorative strip or flag" .)