Band joining river vessel (5)
I believe the answer is:
liner
'vessel' is the definition.
(liner is a kind of vessel)
'band joining river' is the wordplay.
'band' becomes 'line'.
'joining' means one lot of letters go next to another.
'river' becomes 'r' (abbreviation for river).
'line'+'r'='LINER'
(Other definitions for liner that I've seen before include "QE2, perhaps" , "bin-bag?" , "Cosmetic; fine paintbrush" , "Material applied to inner surface" , "Seagoing vessel - goes with bin, eye or hard" .)