Film a vessel joining a river (6)
I believe the answer is:
avatar
'film' is the definition.
(I've seen this before)
'a vessel joining a river' is the wordplay.
'vessel' becomes 'vat' (vat is a kind of vessel).
'joining' is a charade indicator (letters next to each other).
'river' becomes 'r' (abbreviation for river).
'a'+'vat'+'a'+'r'='AVATAR'
(Other definitions for avatar that I've seen before include "High-grossing James Cameron science fiction film" , "Personal online icon" , "Screen icon" , "Graphic" , "Manifestation of a deity; cyberspace icon" .)