Pennant shown by vessel? That's about right (8)
I believe the answer is:
streamer
'pennant' is the definition.
(I know that pennant can be written as streamer)
'by vessel? that's about right' is the wordplay.
'by' says to put letters next to each other.
'vessel?' becomes 'reamer' (I am not sure about this - if you are sure you should believe this answer much more).
'right' suggests the final letters (the letter on the right of the word).
The final letters of 'thats about' are 'st'.
'reamer' put after 'st' is 'STREAMER'.
'shown' acts as a link.
Can you help me to learn more?
(Other definitions for streamer that I've seen before include "Long flag -- bad cold" , "Paper ribbon - thin flag" , "Long strip used at parties" , "It blows in the wind" , "Item of bunting" .)