Surrounded by flounder, you caught river fish (7)
I believe the answer is:
grouper
'fish' is the definition.
(I know that grouper is a type of sea bass)
'surrounded by flounder you caught river' is the wordplay.
'surrounded by' means one lot of letters goes inside another (I've seen this in other clues).
'flounder' becomes 'grope' (I've seen this before).
'you' becomes 'u' (commonly used in online communication).
'caught river' becomes 'r' (I can't justify this - if you can you should give a lot more credence to this answer).
'grope' placed around 'u' is 'groupe'.
'groupe'+'r'='GROUPER'
(Other definitions for grouper that I've seen before include "A fish" , "that by implication isn't sole?" , "I order" .)