It is considered a river goes to the sea (6)
I believe the answer is:
deemed
'it is considered' is the definition.
(deeming is a kind of considering)
'a river goes to the sea' is the wordplay.
'a river' becomes 'dee' (River Dee).
'goes to' is a charade indicator (letters next to each other).
'the sea' becomes 'med' (abbreviation of Mediterranean).
'dee'+'med'='DEEMED'
(Other definitions for deemed that I've seen before include "Regarded, considered" , "Viewed as, reckoned" , "Considered, held as opinion" , "Considered, believed" , "Considered, held the view" .)