Time before river revealed fish (5)
I believe the answer is:
trout
'fish' is the definition.
(I know that trout is a type of fish)
'time before river revealed' is the wordplay.
'time' becomes 't' (abbreviation).
'before' says to put letters next to each other.
'river' becomes 'r' (abbreviation).
'revealed' becomes 'out' (as in news which is out).
't'+'r'+'out'='TROUT'
(Other definitions for trout that I've seen before include "Tasty fish, perhaps of rainbow kind" , "old bag" , "tickled to death?" , "Mistiness" , "Small salmonlike fish" .)