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' means one lot of letters go next to another.
'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" .)