Flag in vessel crossing river (8)
I believe the answer is:
streamer
'flag' is the definition.
(I know that streamer is a type of flag)
'in vessel crossing river' is the wordplay.
'in' means one lot of letters goes inside another.
'vessel' becomes 'steamer' (steamer is a kind of vessel).
'crossing river' becomes 'r' (abbreviation for river. I am not sure about the 'crossing' bit.).
'steamer' placed around 'r' is 'STREAMER'.
Can you help me to learn more?
(Other definitions for streamer that I've seen before include "Long flag, often tapering" , "that hangs about now?" , "Long decorative strip of material" , "shocking cold?" , "Long strip used at parties" .)