Flag flown by ship crossing river (8)
I believe the answer is:
streamer
'flag flown' is the definition.
(streamer is a type of flag)
'ship crossing river' is the wordplay.
'ship' becomes 'steamer' (steamer is a kind of ship).
'crossing' is an insertion indicator.
'river' becomes 'r'.
'steamer' enclosing 'r' is 'STREAMER'.
'by' is the link.
(Other definitions for streamer that I've seen before include "Long flag -- bad cold" , "Ribbon for throwing" , "awful cold?" , "Long narrow ribbon for er, master" , "Long decorative ribbon - thin flag" .)