Every character by river is blue (5)
I believe the answer is:
azure
'is blue' is the definition.
(I've seen this before)
'every character by river' is the wordplay.
'every character' becomes 'az' (every letter (character) from A to Z).
'by' is a charade indicator (letters next to each other).
'river' becomes 'ure' (River Ure).
'az'+'ure'='AZURE'
(Other definitions for azure that I've seen before include "Sort of blue" , "heraldic colour?" , "A bright sky-blue" , "Blue like the sky" , "Bright blue" .)