Behind river, exposed fish (3,5)
I believe the answer is:
sea trout
'fish' is the definition.
(I know that sea trout is a type of trout)
'behind river exposed' is the wordplay.
'behind' becomes 'seat' (synonyms).
'river' becomes 'r' (abbreviation for river).
'exposed' becomes 'out' (out on display).
'seat'+'r'+'out'='SEA-TROUT'
(Other definitions for sea trout that I've seen before include "Salmon-like fish that also migrates from salt to fresh water" , "Something fishy here" , "One angled" .)