Fish was circling river and most of ocean (6)
I believe the answer is:
wrasse
'fish' is the definition.
(wrasse is a kind of fish)
'was circling river and most of ocean' is the wordplay.
'circling' indicates putting letters inside (circling can mean going around or containing).
'river' becomes 'r'.
'and' is a charade indicator (letters next to each other).
'most of' means to remove the last letter.
'ocean' becomes 'sea' ('sea' can be a synonym of 'ocean').
'was' going around 'r' is 'wras'.
'sea' with its last letter taken away is 'se'.
'wras'+'se'='WRASSE'
(Other definitions for wrasse that I've seen before include "Type of fish" , "Bright marine fish" , "catch in the deep" , "Brightly-coloured fish" , "Swimmer" .)