Mean to cross river with a fish (8)
I believe the answer is:
stingray
'fish' is the definition.
(stingray is a kind of fish)
'mean to cross river with a' is the wordplay.
'mean' becomes 'stingy' (similar in meaning).
'to cross' indicates putting letters inside.
'river' becomes 'r' (abbreviation for river).
'with' is a charade indicator (letters next to each other).
'r'+'a'='ra'
'stingy' enclosing 'ra' is 'STINGRAY'.
(Other definitions for stingray that I've seen before include "swimmer" , "Large fish with tail spine" , "Fish with venomous tail" , "Dangerous fish" , "TV show" .)