Fish river bottom in case of wave (6)
I believe the answer is:
wrasse
'fish' is the definition.
(wrasse is a kind of fish)
'river bottom in case of wave' is the wordplay.
'river' becomes 'r'.
'bottom' becomes 'ass' ('ass' can be a synonym of 'bottom'**).
'in' indicates putting letters inside.
'case of' suggests removing the centre (outsides of).
'wave' with its middle removed is 'we'.
'r'+'ass'='rass'
'rass' inserted within 'we' is 'WRASSE'.
(Other definitions for wrasse that I've seen before include "Bright marine fish" , "Swears about the tropical marine fish with strong teeth" , "Type of fish" , "Brightly-coloured fish" , "catch in the deep" .)