All characters by river are blue (5)
I believe the answer is:
azure
'are blue' is the definition.
(I've seen this before)
'all characters by river' is the wordplay.
'all characters' becomes 'az' (I can't explain this - if you can you should give a lot more credence to this answer).
'by' means one lot of letters go next to another.
'river' becomes 'ure' (river in Yorkshire).
'az'+'ure'='AZURE'
Can you help me to learn more?
(Other definitions for azure that I've seen before include "Description of sky, perhaps" , "A bright sky-blue" , "Cerulean" , "heraldic colour?" , "Deep blue colour" .)